Indigo is a color that sits between blue and violet on the visible spectrum. It has a longer wavelength than violet but a shorter wavelength than blue. This places indigo in an ambiguous position between being considered a shade of blue or being its own distinct color. There is much debate around whether indigo should be classified as blue or as its own unique hue.
The History of Indigo
The history of indigo dates back thousands of years. In ancient times, indigo dye was derived from the indigo plant and was one of the earliest dyes used for textile dyeing and printing. The earliest known uses of indigo as a blue dye come from 6th century BC Peru. Indigo was also used in ancient Egypt and was imported via trade routes to the Mediterranean. By the 12th century, indigo dyeing had spread to India, which became the world’s primary indigo producer up until the 19th century.
The term “indigo” itself has unclear origins. It may have originated from the Greek word indikon meaning “‘blue dye from India.” The Latin term indicum also referred to the indigo plant grown in India. Early Arabic sources used the word nili to describe the color. In various languages, indigo was described as blue or purple, linking it to those shades rather than viewing it as distinctly separate.
When Isaac Newton developed his color wheel in 1666, he placed indigo as one of the seven fundamental colors of the rainbow. At the time, the color spectrum was defined as red, orange, yellow, green, blue, indigo, and violet. This cemented indigo’s place as a distinct color next to blue on the visible color spectrum.
How Indigo Differs from Blue
Indigo sits between violet and blue on the visible spectrum. Violet light has shorter wavelengths and higher frequencies than indigo, while blue light has longer wavelengths and lower frequencies. This means indigo reflects higher frequency light than blue.
The wavelengths of light for different colors are:
- Violet: 380-450 nm
- Indigo: 445-475 nm
- Blue: 475-495 nm
When comparing blue and indigo paint pigments, indigo often contains more violet undertones than traditional blue pigments. This gives it a deeper, richer color than pure blue.
Indigo dye was historically extracted from the indigo plant, while blue dyes were extracted from other sources like woad or lapis lazuli. Having an independent plant source contributed to indigo being viewed as a distinct color from blue.
The differentiation of indigo from blue in various world languages and trade also indicates it developed independently from blue rather than being considered a mere shade:
- In Japanese, indigo is referred to as ai while blue is ao
- In Portuguese, anil describes indigo while azul is blue
- In Hebrew, techelet is indigo and kakhol is blue
Modern color science and optics does classify indigo as a spectral color with it’s own specific wavelength range in the color spectrum. This provides some scientific basis for indigo being considered separate from blue.
Similarities Between Blue and Indigo
While indigo can be differentiated from blue due to it’s technical properties, there are also many similarities between the two colors:
- They are both considered cool colors and sit next to each other on the color wheel
- They both have hue angles between 240° and 280°
- Mixing blue and indigo together creates a variant of blue, demonstrating their close relationship
- Blue is primary subtractive color, while indigo can be approximated by mixing blue and violet paints
- Some shades of indigo are perceptually difficult to distinguish from shades of blue
Due to the close proximity of blue and indigo wavelengths, the human eye often struggles to perceive the difference unless colors are viewed side by side. When presented alone, indigo will usually appear similar to blue for most observers.
In printing and digital displays, indigo is usually simulated using combinations of blue and violet light rather than being reproduced exactly. This causes it to be perceived as a variant of blue rather than as distinctly separate from blue hues.
Indigo Classifications
Whether indigo is classified as blue depends on the color system being used:
- Natural Color System – Classifies indigo as a hue of blue
- Munsell Color System – Places indigo with blue hues
- CIE 1931 Color Space – Indigo wavelengths defined separately from blue
- HSB/HSV – Considers indigo to be a shade of blue
- RYB – Primary color system with indigo as separate hue
Some of the confusion arises from the difference between additive and subtractive color systems. Additive systems like RGB place indigo with blue, while subtractive systems like RYB define it independently.
| Color System | Classification |
|---|---|
| RYB | Separate color |
| RGB | Shade of blue |
| CMYK | Mixture of blue and violet |
| HSB/HSV | Blue hue |
This demonstrates how indigo falls into a gray area, sometimes classified as a distinct secondary color and sometimes considered a primary blue hue depending on the color system.
Indigo in Optics
In the field of optics, indigo is technically defined as its own separate color based on its wavelength. Isaac Newton originally identified it as a spectral hue next to blue when he passed sunlight through a prism and saw the separation of colors.
Modern color scientists measure indigo’s wavelength as between 445 and 475 nanometers. This separates it from the blue wavelength of between 475 and 495 nanometers on the visible color spectrum. In technical terms, indigo photons oscillate at a distinctly higher frequency than blue photons.
However, the human eye does not contain receptors that can differentiate indigo wavelengths from blue. Humans have three types of color receptors: red, green, and blue. There are no indigo receptors. This means that although indigo has a separate wavelength, we perceive it as a variant of blue.
Indigo and Color Vision
The trichromatic theory of human color vision states that the eye uses blue, red, and green receptors to see the full spectrum of colors through combinations of signals from these three receptor types. There is no neurobiological evidence for people having unique indigo photoreceptors.
Tetrachromatic vision has been documented in some rare individuals, mainly women. This allows the eye to perceive around 100 million more chromaticities through having four independent channels for color information. Even in tetrachromats however, unique indigo photoreceptors have not been found.
This helps explain why most people have difficulty distinguishing indigo from blue: they lack retinal cells that can independently detect indigo wavelengths. From a biological view, indigo vision is processed by the blue retinal receptors and neural circuits.
Indigo in Art
In the art world, indigo has historically been considered its own distinct hue rather than a variant of basic blue.
Impressionist painters like Van Gogh and Monet used the term “bleu d’indigo” to refer to the deep blue indigo pigment in their paintings. They distinguished this from “bleu de cobalt” which was a brighter blue pigment.
Contemporary artist Yves Klein worked extensively with monochrome indigo canvases which he trademarked as International Klein Blue. He choose indigo for it’s rarity, depth, and distinction from other blues.
In fiber arts, weaving, and dyework, indigo holds particular cultural significance and has its own unique dyeing traditions. Japanese shibori artists focus extensively on indigo patterns and designs that differ from other blue dyes.
Modern color theory does make a perceptual distinction between blues which are seen as tranquil and cool vs indigo which has a warmer, deeper tone. Indigo evokes richer emotional responses more similar to violet than blue.
Psycological Effects of Indigo
Psychologically, indigo is associated with serenity, intuition, inner calm, and personal insight. It is thought to aid meditation, introspection, and spiritual growth. This differs from the cold detachment of blue.
One psychological study found that people were able to recognize the unique properties of indigo even if they could not explicitly name the color as indigo. The participants viewed indigo as having more red, purple, and pink tones than blue samples.
Indigo can have a mystical or sacred essence, hence it’s use in many religious and spiritual rituals throughout history. This spiritual dimension sets indigo as its own color distinct from more mundane blues.
Gender Associations
From a gender perspective, indigo is more often categorized as a masculine color while blue is seen as feminine. Researchers suggest this may come from indigo being associated with authority figures and religious officials who were historically male dominated.
Blue is strongly associated with calmness, empathy, and nurturing which are feminine stereotypes. So while blue and indigo are similar hues, cultural and gender meanings attached to each set them apart psychologically.
Some cultures also saw indigo as representing masculinity and strength due to its rarity compared to other dyes. The labor intensive process for true indigo dye lent it a masculine workmanship.
Use in Fashion
In the fashion world, indigo holds unique cultural significance. Denim jeans were traditionally dyed with indigo, making it the ubiquitous color of everyday workwear. This cemented indigo’s reputation as an authentic, rugged hue associated with physical labor.
Indigo garments take on a symbolic personality unlike factory produced blue jeans. Traditional indigo dying methods impart a sense of craftsmanship and cultural heritage that synthetic blue dyes lack.
Some high fashion designers will specify indigo wash processes to get these unique cultural connotations compared to regular blue denim. So although the color difference may be subtle, indigo offers distinct stylistic messaging.
Digital Representations
Digitally representing indigo poses challenges due to the limited color gamut of RGB computer displays. They cannot accurately reproduce the spectral indigo wavelength.
By mixing RGB components, computers can simulate an approximation of indigo. But this loses the spectral purity that defines indigo as separate from blue. It becomes blurred into a generic digital blue.
Some examples of RGB indigo representations are:
| Color | RGB Values |
|---|---|
| Electric Indigo | 49, 46, 129 |
| Indigo (Crayola) | 75, 0, 130 |
| Blue-Violet | 138, 43, 226 |
While these can simulate indigo, they miss out on the spectral purity that defines indigo as a distinct secondary color rather than a variant of primary blue.
Indigo Dyeing Techniques
What sets indigo dyeing apart from other blue dyes is it’s labor intensive artistic processes. Traditional methods involve meticulous steps of fermenting, precipitating, aerating, and dipping to build up layers of indigo pigment.
The natural indigo compound indican produces a clear yellow-green solution. To form insoluble blue indigo pigment, the indoxyl has to undergo oxidation. Traditional indigo vats controlled this process through fermentation.
Some traditional indigo dyeing techniques include:
- Shibori – Japanese resist dyeing using stencils and wraps
- Wax resist – Applying wax to block dye uptake
- Clamp dyeing – Clamping and pleating fabric
- Vat dipping – Dipping cloth in reduced indigo vat
Artisans intricately control the dye process to create patterns. The craft involved elevates indigo as special compared to mechanically produced blue dyes.
Natural vs Synthetic Indigo
Natural indigo has a unique chemical signature compared to synthetic indigo. Traditional techniques carefully orchestrate solubilizing indican precursor from plant matter into insoluble indigo pigment.
Synthetic indigo simply involves mixing prepared pigments. This allows mass production of blue jeans unattainable with traditional natural indigo vat dyeing. However, the cultural heritage is diminished.
Even once dyed, natural and synthetic indigo can be differentiated. Analytical chemistry can identify characteristic impurities and trace compounds lacking in synthetic indigo.
This further separates natural indigo as a distinct ingredient with unique cultural impact compared to industrial synthetic blue dyes.
Conclusion
While physically indigo has a wavelength between violet and blue, psychologically it retains a distinct identity from blue. Indigo dyeing techniques impart cultural depth unmatched by synthetic blue dyes. The color carries gender, artistic, and spiritual meaning that differs from blue.
Indigo’s technical definition supports it as a separate spectral color from blue. But biologically, human vision perceives it as a variant of blue due to lack of indigo retinal receptors. Ultimately, indigo rests in an ambiguous region both technically and perceptually. It possesses an identity separate from blue, but still exists in proximity to and intertwined with blue in a cultural, biological, and aesthetic sense.
