In traditional color theory, there is the color wheel. The color wheel is nice and symmetric. But have you ever wondered why yellow seems brighter than the other colors? Why, when the color wheel is converted to gray-scale, isn’t it a uniform gray?
Two RGB color wheels are shown, but the right one is converted to grayscale.
First, the glib answers:
- Yellow gets converted to the lightest gray because I picked a gray-scale conversion that does that. (But why did I pick that particular gray-scale conversion?)
- Yellow comes in dark variants, but we don’t call those colors yellow. (But why don’t we call them yellow?)
Ultimately, the explanation is that our eyes are simply more sensitive to yellow light.
Light, of course, doesn’t have any intrinsic color. Light simply has wavelength, or a distribution of wavelengths. Our eyes have three types of color sensors (“cones”), one which is most sensitive to blue, one which is most sensitive to green, and one which is most sensitive to red. Color is psychologically constructed from the relative stimulation of different cone types.
The sensitivity of the three types of cones as a function of light wavelength. In this graph, the three cones are shown on a similar scale, but in fact they are not equally sensitive, and you do not have equal numbers of each cone type. Image credit: Wikimedia Commons
We must distinguish the brightness of light and its luminance. Brightness refers to the actual physical power within the light, and luminance is the sensation of there being more light. Different wavelengths of light have different luminous efficiency, meaning that the same brightness corresponds to different amount of luminance. In the average human eye, yellow light (555 nm) has the greatest luminous efficiency.
In the image at the top, I personally balanced the gray-scale filter to match my own sense of luminance on my own computer screen. I’d be interested to hear if your own sense of luminance is different. You might have more or less of one of the cone types compared to me. I’m particularly interested to hear from any colorblind people.
My image software indicates that this is how the colors would look with with protanopia (left) and deuteranopia (right). Protanopia means red cones are absent, and deuteranopia means green cones are absent, although I think it’s more common for color blind people to have reduced sensitivity, rather than cone types missing entirely.
The luminance of yellow in my color wheel doesn’t just depend on luminous efficiency though. It also depends on the brightness of the yellow produced by your screen. Standard screens generate colored light by mixing three colors, red, green, and blue. Red and green produce yellow. Note that they must start with some particular shades of red, green, and blue. In fact, this limits the number of colors that can be produced.
The horseshoe shape represents the full space of perceivable colors in CIE xyY coordinate space. The triangle is the set of colors that can be produced on a standard screen. The colors shown are false, since obviously your screen can’t produce the actual colors. D65 is a common, but arbitrary standard of “white”. Image credit: Wikimedia Commons
It would certainly be possible to create a screen where yellow is less bright than the other colors, allowing yellow to have equal luminance to the other colors. But then, we wouldn’t call it yellow anymore, we’d call it olive or brown! Why is that?
Screens emit their own light, but our color vision is adapted to light that is reflected. So our interpretation of color is influenced by the perceived lighting environment. A lemon in the dark might produce light that is similar to an olive in the light, but we don’t perceive those colors as the same.
The reason we view yellow as a canonical color is–again–because we are more sensitive to yellow as compared to other colors. There are many objects, such as lemons, which have high luminance compared to the lighting environment. There are relatively few blue objects which have high luminance compared to their lighting environment. Even if an object reflects all incident blue light, blue has low luminous efficiency, and thus the canonical blue has low luminance.
I hope this color tour has illustrated that colors are not all equal, and that they exhibit a high degree of structure.
Except where otherwise noted, all images in this post are mine. Please give me credit if you borrow them.