Dark grey can sometimes appear to have a blueish tint or hue. This is an optical illusion that is caused by several different factors relating to the way our eyes and brain perceive color. Understanding why this happens requires looking at the properties of color, light, and vision.
There are a few key reasons that dark grey can look blue:
- Surrounding colors – When placed next to or surrounded by warm colors like red, orange, or yellow, a dark grey will appear cooler and bluer by comparison.
- Lighting conditions – Cool, blueish lighting conditions can cause neutral greys to take on a bluer cast.
- Chromatic adaptation – After looking at a bright color for a while, its complement may appear in nearby neutrals. So after looking at orange, neutrals may appear blue.
- Optical mixing – Darker shades absorb more short wavelength blue light, causing an optical mixing effect with the grey.
The illusion of dark greys appearing blue reveals some interesting facets of human color and vision. Let’s explore exactly why this happens.
How We See Color
To understand why dark grey can look blue, we first need to understand some basics about light, color, and human vision.
Visible light consists of different wavelengths across the electromagnetic spectrum that our eyes can detect. This spans colors from violet and blue light with short wavelengths, to orange and red light with longer wavelengths. Our eyes contain special photoreceptor cells called cones that can detect these colors. We have three types of cones – blue, green, and red cones. These cone cells respond to and are stimulated by different wavelengths of light. The pattern of stimulation across our cones allows our visual system to perceive all the colors we see.
When we see a color, it is because that pigment or object is selectively reflecting back a specific band of wavelengths while absorbing the rest. For example, something that appears red reflects long wavelength light strongly while absorbing other colors. Something blue reflects shorter wavelengths while absorbing the rest. Grey is an achromatic or neutral color, meaning it reflects light across the spectrum evenly and does not selectively absorb particular wavelengths.
Surrounding Colors
One of the most important reasons that a dark grey can appear to have a blue tint is the surrounding color context. Our perception of color is strongly influenced by neighboring colors and contrasts. This effect is known as simultaneous contrast.
Put simply, when a color is surrounded by its complement, it will appear even more saturated. The complementary color seems to be induced in the original color to help it stand out against its surround.
So when a dark grey is placed against a strong warm color like red, orange, or yellow, it will appear cooler and bluer than against a neutral backdrop. The blueness helps it contrast against the warmth.
Here is a diagram showing this effect:
| Warm surround | Cool surround |
|---|---|
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The same medium grey square appears tinted blue against the orange backdrop, while appearing dull and brownish against the blue backdrop. This reveals the powerful effect that neighboring colors have on our color perception.
Lighting Conditions
The color temperature of ambient lighting conditions also impacts how we perceive greys and neutrals. Cool, blueish lighting can cause neutral greys to appear bluer than they would under a warm light.
Natural daylight varies in color temperature depending on factors like time of day, weather, and exposure. Early morning or evening light is warmer, while noon daylight is cooler and blueish. Overcast days have a cool, blue cast while sunny days appear warmer.
Indoor lighting also varies widely in its spectral composition. Some common light sources are:
- Incandescent bulbs – warm, reddish light
- Old fluorescent lighting – cool, green/blue cast
- Modern LED lighting – can vary depending on CCT rating
- Candlelight – very warm yellow/orange cast
Under most warm, incandescent lighting, greys will maintain their neutrality. But the cooler the light source, the more blueish greys can become. Our visual system adapts to the ambient lighting and essentially “normalizes” it. So under cool blue light, neutrals look blue until our perception compensates.
Chromatic Adaptation
Closely related to the effect of ambient lighting is the phenomena of chromatic adaptation. This refers to how our visual system adjusts and adapts to different color environments over time.
When exposed to a strong color for a prolonged period, the cones that are most sensitive to that color become desensitized or fatigued. This causes an adaptation effect where we see an “after image” of that color’s complement when looking at neutrals or lighter colors.
For example, if you stare at a bright orange color for 30 seconds or longer, then shift your gaze to a light grey area, it may briefly appear tinted blue before the effect fades. This is caused by the orange color fatiguing your red and green cones, causing their complement (blue) to be induced in the light grey momentarily.
So prolonged exposure to strong warm colors like orange can cause subsequent neutrals to take on a temporary cool, blue cast – an after effect of adapting to the warm color.
Optical Mixing
In addition to these color contrast and adaptation effects, the inherent optical properties of darker neutrals also influence why they can appear blue. This is based on how mixtures of light interact with our vision.
As discussed earlier, a pure grey has a neutral spectral reflectance profile – it reflects all wavelengths of visible light fairly evenly. However, darker shades of grey reflect less light overall across the spectrum. What’s interesting is that this darkness disproportionately absorbs more of the short wavelength blue light compared to the longer red/orange wavelengths.
So in a sense, at darker intensities greys cease to be fully neutral. Darker greys absorb a greater percentage of blue light, resulting in a subtle optical color mixing effect, leaving a slightly warmer, orangey cast behind.
Our visual system seems to overcompensate for this relative loss of blue, causing the grey to appear slightly blueish. So we perceive an illusion of blue in the darker grey when in fact it is absorbing more blue wavelengths.
Blue Perception in Vision
Not only do the properties of colors and light influence how dark greys may appear blue, but also the quirks of human vision itself. Our eyes and visual cortex in the brain evolved with a bias towards being extra sensitive to blue light compared to other colors.
Some theories as to why we perceive blue more strongly:
- Helped early humans better spot sources of water
- Enhanced ability to see through haze and fog
- Supported foraging for berry fruits
So our visual networks seem wired to be particularly receptive to blue wavelengths. This could be another factor exaggerating the subtle effect of darker greys appearing to have a bluish cast.
Individual Variation
While these visual effects cause grey to often appear bluish, the degree varies between individuals. Color perception is subjective and influenced by factors like:
- Genetics – variations in cone photopigments
- Age – yellowing of the lens and macular degeneration
- Environment – difference in ambient color exposure
Younger people with healthy eyes are generally more sensitive to blue light and these chromatic effects. As the lens ages it yellows and absorbs more blue light, reducing the apparent blueness of greys. But overall these illusory effects are experienced to some degree by most normal, healthy vision.
Conclusion
In summary, several factors can lead to the illusion of neutral dark greys appearing to have a bluish tint or cast:
- Simultaneous contrast with surrounding colors
- Cool ambient lighting conditions
- Chromatic adaptation and afterimages
- Uneven absorption of bluish wavelengths in darker shades
- Enhanced blue sensitivity in human vision
This reveals some interesting aspects of color theory, light physics, and visual neuroscience. While counterintuitive, it is a common and perceptually real effect under the right viewing circumstances. The blueness is an illusion, but demonstrates the complex ways that colors can interact and influence each other in human vision.
