Why do colors illuminated with moonlight lack color?
The quick answer is: they don't. Light reflected to the Earth from the Moon's surface is composed of colors throughout the visible spectrum. Thus the surface (of whatever color) does infact reflect various colors of light back to your eye. However it is true that under most circumstances we do not perceive the color of moonlit objects.
This effect is not due to the spectral composition of the light, but the intensity of it. Our retinas contain two discrete classes of photoreceptors: rods and cones. Cones come in three types, one which responds strongly to each of the three primary colors of light. Thus our color vision arises; by comparing the output of the three different cone types, the human eye can discriminate between many thousands of distinct hues and tens of thousands of different colors. However all of the cones require fairly intense radiation to be stimulated at all.
Rods on the other hand, do not respond much differently to different wavelengths of light, and so provide what we think of as 'black and white' vision. But the rods are much more sensitive to photons, and so will still work under low light conditions (especially when 'dark adapted'.
Thus, the low light conditions of night vision usually do not stimulate cones, and all we are left with is a colorless perception of the world around us.
Rob Landolfi, Science Teacher, Washington, DC
The primary reason is due to the way our vision system is designed. Our eyes are a high-resolution color sensing system with a wide dynamic range. This means we can see a broad range of colors and sharply resolve images with a wide variation in brightness - a miraculous combination.
The parts of our eyes that are sensitive to color are called cones. They function primarily in the bright sunlight and work like a three color image scanner. The parts of our eyes that provide high resolution and sensitivity to a wide range of intensity or brightness are called cones.
Our color-sensing rods require a high level of illumination to function properly. Cones, that can discern shapes and texture in very low light levels, are not sensitive to color.
Even though there are colors reflected from objects illuminated by moonlight, they are just too dim to activate our color vision.
See http://www.photo.net/photo/edscott/vis00010.htm for a detailed explanation of human vision (in color).
Scott Wilber, President, ComScire - Quantum World Corporation
'Our job in physics is to see things simply, to understand a great many complicated phenomena, in terms of a few simple principles.'