Any publication about color should start with a definition of color. There are nearly as many ideas about what color is as there are people on this earth. So in order clarify what is meant with the term ‘color’ on these pages, here’s a definition:

‘Color is an event that occurs among three participants: a lightsource, an object and an observer.’

I first came across this definition in the book ‘Real World Color Management’ by Bruce Fraser, Chris Murphy and Fred Bunting. It is by far the most usefull definition I ever read because it nicely touches all the things important about color.
Color is an event because it involves an occurrence, something that happens at a certain time in a certain place. The light is involved because it is the ‘carrier’ of color. Without light there’s no color. Light may exist without color, but in that case it’s invisable. Just look up on a starry night and you will notice that the space between the stars is pitchblack. Yet we can be sure all that all visible stars fill that space with light. But because the light doesn’t bounce off on anything, doesn’t meet any surface, it remains invisable to us and without meaning.
The second participant in the color event is surface. The surface is what most people actually associate with the term color. The surface may be for instance a layer of paint, paper or skin.
Lastly the observer. The observer is often neglected but is nevertheless very important. The observer very much determines the color event. Eyes may differ in sensitivity from one person to the next. But much more influential is what goes on in the observer’s brain: color constancy, perceptual order etc. In the brain of the observer, white may be seen as black, black may be seen as white, red may be seen as green. It is the brain of the observer that keeps insisting on ‘seeing’ paper white, even though the color may not be white at all due to lighting, when entering the eye. You don’t believe white may be black? Here’s a picture by Edward Adelson that may convince you:

I actually once asked Fred Bunting about the origin of the afore mentioned definition. Not willing to take full credit, he traced it back to various philosophers as well as his own work at Pixar studios. Nevertheless, he probably was the first to put it in this exact phrasing.