When I comment on other people’s work, digital work in particular, one of the most frequent phrases I use is “Do not use neutral gray together with color”. Unless you know exactly what you are doing, don’t ever use neutral gray together with color. It is too easy to get a neutral gray on the computer (fortunately, it’s not as easy with physical media), and its effect when used as if it were any other color is disastrous. But, since knowledge cannot be harmful, and in light of the sayings “Rules are made to be broken” and “Know thy enemy”, I am going to look at neutral gray and its cousins in detail, to show exactly why it does not live well with color. I will also discuss what can be done to fix problems with neutral gray, what subtle traps lie beneath the quiet water, and, ultimately, when and how to break the rules.
Science of color
We begin with understanding how color works, with solid, measurable, verified facts. The perception of color is determined by physics and physiology: it is the eye’s perception of the brightness and the frequency spectrum of light. Both the brightness and spectrum of an object depend primarily on the quality of the colored object’s surface, with the added influence of the brightness and spectrum of the light itself. In art, this brightness is usually called tone or value, and the spectral quality is called hue.
Pure colors have a very narrow spectrum: their brightest part is very close to a single frequency (Laser light is the extreme, since it is a single frequency.). Due to the nature of color perception, the wider is the frequency range of a color, the grayer it looks - until its spectrum covers the complete range of visible light and it becomes white. The three kinds of receptors in the eye are tuned to the low-frequency end of the range, the middle, and the high frequency end, and their signals are processed by their intensity relative to each other. Thus, when all three receptors receive the same intensity of light, this is perceived as white. In the mathematical theory of color, the width of spectrum is linked to the third parameter, the saturation. The closer the intensities of the signals picked up by the receptors become, the wider the spectrum is, and the less saturated the color is. White is completely unsaturated, and it has no hue, since it does not have any peaks at any distinct frequency!
The brightness of the light, on the other hand, is perceived without regard for its frequency. It influences the signal level of the eye receptors directly. No matter how bright the laser light becomes, it will never seem white - it will only be very bright pure color. As the brightness decreases, the color gradually descends toward black, which is simply how we perceive the absence of light. The absence of red looks exactly the same as the absence of white! Black is also unsaturated, and has no hue, since brightness itself is just the amount of light, and does not depend on its frequency in any way.
So color is really two channels of information, due to the way our eyes are built: frequency information and intensity information. Artists refer to these channels as chromatic and achromatic scales: the chromatic tones carry frequency information (i.e. their spectrum is uneven) the achromatic tones carry brightness.
Brightness perception is more basic than frequency perception; most animals only see brightness. The spectrograph in our eye is a much later evolutionary acquisition. It could be compared to music: the brightness is the rhythm, the hue a direct analogue of the scale. Just as musical rhythm provides the basic organization to the ear, the brightness levels organize the visual image. Hue and musical scale only add another dimension: just as one can make pure-rhythm music on a drum, one can make pure-brightness images with black and white. (I avoid the term “tone” here, because its meaning in music is unfortunately the opposite from its meaning in color science; musical tone is analogue of hue.)
Gray in nature
Colors become progressively more gray as their frequency range widens (saturation decreases). Eventually, there is so little distinct hue perceived in them that we perceive them as gray. However, there are many types of gray: there is the greenish gray of concrete, and the yellowish gray of limestone, and the bluish gray of slate, and so on. In fact, most real world objects transmit or reflect a rather wide frequency range (that is to say, they all are more or less “gray”). Completely saturated colors are rare–but completely gray objects do not exist.
That’s right-you will never see a pure gray in the world. Even if you made something that reflected the complete spectrum evenly, it would not be completely gray simply because light tends to be colored: the Sun’s “white” is yellowish, the sky’s light is bluish, so even our hypothetical case would be slightly tinted. If something colored is put next to our pure gray object, the diffuse light reflected off that object would tint it, again. The eye also does tricks with perception of juxtaposed colors: it is a side effect of the neural scheme that enhances contrast, so a color next to red will look greener than the same color next to white, and redder next to green.
This finally allows us to understand what a neutral gray is: it is just another name for the “colors” of the achromatic scale. It is not, strictly speaking, a color at all, since it lacks one of the two components of color: neither white nor black carries any hue information, and neither do their mixtures. If you add white paint to any color, that would increase its brightness and somewhat widen its frequency range - desaturate it. If you add black, that would decrease the brightness, and also desaturate it. Brightness is the only thing white or black can contribute to a color; frequency information, they can only take away, making the frequency range wider and less distinct. (Information is always in the differences. A uniform spectrum carries none, and is also known as white noise - the term “white” is entirely not coincidental here!) Mixing white and black can only produce pure, colorless brightness, which interacts with nearby colors in a different way than other chromatic hues would do.
