I was reading through Josef Albers' interaction of color (a classical art color theory book), where he makes this point:
If I see a substance, or pigment of a particular color, doesn't that mean I'm seeing reflections of primarily that wavelength that corresponds to that color?
I don't get this point that it consists of all other colors like white.
Color happens in the mind. Light has a mix of wavelengths, but it doesn't have color until you see it.
The color you see is determined by the wavelengths in the light that reaches your eyes. And that is determined by the wavelengths you start with and which wavelength are absorbed by reflection.
You see white from a mix of all wavelengths. Red is from just the longer wavelengths. Blue is from just the shorter wavelengths.
You shine white light on an object. Suppose it absorbs short wavelengths and reflects long wavelengths. Now all that is left is long wavelengths. You see red. We say that an object like this is red. It turns white light red.
Suppose you shine blue light on that object. It absorbs the short wavelengths, leaving nothing. It looks black under blue light. We still say it is a red object. The color of an object is determined by its reflectance properties.
This is a simplified picture. We have three types of color receptors in the eye. They are broadly sensitive to short, medium, and long wavelengths, though there is a lot of overlap. Light with just one medium wavelength will stimulate all three receptors, but mostly the middle one.
Light with a mix of long wavelengths will stimulate the long wavelength receptors most. This makes us see red. Other mixes stimulate the receptors differently. The mix of stimulation is how we see all the colors.
There are 3 primary colors because we have 3 types of receptor.
From a practical perspective, a colored object will reflect light at all wavelengths but of course mostly red. And while you can illuminate an object with light of a pure color with a single wavelength and get back reflected light of that wavelength [*], in practice we do not illuminate objects with pure light wavelengths very often - there is usually a broad spectrum not just of the illumination source, but also objects around it casting ambient light and reflections onto it.
[*] Some objects may also turn reflect light of one wavelength into light of several wavelengths (e.g. dichroism or fluorescence). But this is uncommon in everyday life.
