Is nature colorless?
There are colors, and physicists talk about them in terms of light spectra. More precisely, physics knows and studies the spectrum of light, and part of the spectrum is perceived as colors. In fact, the spectrum is even richer than what outr brain makes of it when reducing the many possible frequencies to a just 2-dimensional color space printed in typical color maps, or 3-dimensional, if you add the brightness scale.
Using quantum mechanics, one can physically predict which color will be seen by normal persons when they look at a physical system. For example, one can use quantum mechanical techniques to correctly determine the color of gold, just given the number of electrons of a gold atom and the mass of its nucleus.
In this sense, color is an objective phenomenon, not an illusion.
Only how it ''feels and looks'' to see a color is subjective to
people and their brain, and cannot be studied in terms of physics.
In fact, it is impossible to even know whether you perceive greenness
in the same way as I. Limited information about that can be obtained
through interrogation, but this is not an acceptable method for
obtaining physical information. But from this we know at least that
some people are colorblind and don't perceive greenness at all.
And we can compare whether certain shades of color can be
differentiated by which people (artists usually can differentiate far
more than untrained people), and many other things that are objective
in a more limited, but nonphysical sense.
Arnold Neumaier (Arnold.Neumaier@univie.ac.at) A theoretical physics FAQ