Tone Color: Tone color or timbre of a sound is determined by the relative strengths, presence, and absence of the different harmonics and frequencies that a sound is composed of. On the guitar, if we emphasize the higher frequencies, a note sounds brighter. If we emphasize the mid-range and lower frequencies, it sounds darker. The manner in which you pluck a string determines which frequencies are amplified and which frequencies are diminished.
Simpler Definition: A brighter color tone can be played closer to the bridge while a darker color tone can be played over the sound hole.
Video Explanations:
Video: What is Tone Color? (Timbre)
Video: What are Color Tones and how to use them
Written Articles:
Article: Tone Color and The Classical Guitar
Fifty Favorite Effect Pedals for Sale: