Color In The Digital Arts- Part 2
Additive Color And Subtractive Color
Color is a completely natural response of our visual cortex to our environment. We don’t have to think about it, it is just there. The problems arise when we want to represent or reproduce color, which is fundamental to any kind of visual design. When I take out my 48-pack of crayons to draw a tree trunk, is it brown, or is it raw sienna, or raw umber, or even burnt sienna, what the heck is that? These names are not arbitrary, but they don’t seem to fit neatly into the rainbow that is the visible spectrum. Digital technology offers the potential for a very precise specification of color, but first we need to understand two fundamental color models, or ways of representing color. One mixes light, the other mixes pigments.
Let’s start with first principles. The retina of each of our eyes has around six million cones that are sensitive to specific ranges of wavelengths within the visual spectrum, some to the reds, some to the greens and some to the blues. The visual cortex responds to the total mix of stimuli from the cones and generates the sensation of color that we see.
We call this an additive color system because the individual stimuli from the cones are added together. This has some profound implications. We have no “purple” cones, for example, but the right mix of stimuli (a lot of blue, two thirds as much red, and a dash of green) will consistently generate the sensation of purple. Light at a wavelength of 650 nanometers will generate the sensation of orange, but the right mix of yellow (600 nanometers) and red (700 nanometers) will generate exactly the same sensation, even though there is no longer any 650 nanometer light present.
If we design a device that can emit various mixes of red, green and blue wavelengths, we can reproduce just about any visible color[i]. This is how televisions work, as well as computer monitors and digital cameras. They all emit or receive mixtures of red, green and blue light. Referring to the diagram above, no light at all gives us black (duh!), a mix of red and green gives us yellow, a mix of red, green and blue gives us white and so on. The three basic colors are called primaries, and mixing any two of the primaries in equal amounts gives us a secondary. The secondaries, by the way, are yellow, magenta and cyan.
Most things that we look at do not emit light; they only reflect light. An orange gets its color from the light that lets us see it, and we can verify this by taking it into a dark closet. Presto! It is no longer orange. This is not to say that the orange is entirely innocent in the matter of color. It looks orange because it absorbs the red, yellow, green, blue, indigo and violet wavelengths of the spectrum and only reflects the orange wavelengths. We call this subtractive color.
Unripe tomatoes absorb most wavelengths of light except green, which they reflect. A ripe tomato has undergone a chemical change that makes it strongly reflect red, signaling that it is good to eat (which is how a tomato gets its seeds into circulation). As shown above, though, if we illuminate the tomato with light that has had the red wavelengths filtered out; there is no red available to be reflected, and the tomato looks dark green. The unripe tomatoes also appear “greener” without red light, demonstrating that they too would normally reflect a small amount of red.
Objects that reflect light generate the sensation of color by the way they interact with light, and this depends on what we can call their pigments. When we want to reproduce color in a similar, non-light-emitting way, as on the page of a magazine, we also use pigments, specifically yellow, magenta and cyan pigments in the form of semi-transparent inks.
Here’s the subtractive color model. The default situation is now white; if we don’t put any pigment down, light is reflected unchanged. Put down cyan pigment, we see cyan (blue-green). Put down a mixture of cyan and magenta (blue-red) and we see blue. Put down equal amounts of cyan, magenta and yellow and we see black.[ii] Notice how the subtractive primaries, yellow, magenta and cyan, generate red, green and blue as their secondaries.
In order to get the subtractive primaries to “mix” on a printed page, we break them down into dots that are barely visible to the naked eye. We can adjust the “strength” of each primary by adjusting the size of its dot, and as shown above the colors are blended together by the visual cortex to generate the sensation of a single color, as long as the dots are small enough.
To summarize, additive color is formed by emitted light, subtractive color is formed by reflected light. To distinguish between the two, just apply the dark closet test. Sun, lightning, molten lava, television: all additive. Orange, beer label, man-eating tiger: all subtractive. Remember that our eyes run in additive mode, and that we can generate all colors from just three primaries because that’s exactly how our eyes do it. The color sensation generated by the visual cortex is a response to a specific mix of red, green and blue signals received from the retina.
In the next part of this blog we’ll look at how color is measured in different color models.
[i] A typical additive color device will have minor limitations in representing colors like deep blues and bright greens, but this is because of the practical limits imposed by the technology.
[ii] In practice, because pigments can never be 100% pure, we see a dark brown, so in printing a small amount of black is added to “key” the image, giving us the CMYK color system.