Here's a definition of color that has proven to be quite usefull to me:
‘Color is an event that occurs among three participants: a lightsource, an object and an observer.’
This definition was introduced in the book Real World Color Management by Chris Murphy, Fred Bunting and the late Bruce Fraser
I once asked Fred Bunting about the origin of this definition. He pointed at several philosophers, but also particularly at Pixar Studios, where he used to work. I very much like this definition because it involves all elements of color that are important to the designer:
The lightsource is obviously important because without light, there’s no color. At least not in my eyes. Moreover, the lightsource very much determines the quality of the color. See my recent blogpost about the Taj Mahal.
The object, or rather the surface of the object, is what most people associate with color. So that would be the layer of paint, the cladding of a building.
And finally, much underestimated, the importance of the observer. Who is looking at the color? How is the color perceived? After all, our eyes are only the outer parts of our visual system. All the processing and interpretation, all the actual ‘seeing’ if you like, is performed under the skull.