My personal guess is when you strike a string, a wave travels back and forth along it, albeit extremely rapidly. When you hit the string again, there will be subtle variations in the tone depending on how the new wave propagates in the string and interacts with a previous one. This may even be true without the damper pedal being depressed as some energy is still in the string even as it is being silenced, I guess, not to mention cross-resonance with sounds already produced. So I'm guessing, variation in color of tone can be quite vast, but this is primarily due to timing. All of the psychological pedagogy about how to approach striking keys are probably effective ways, once practiced, of producing a personal set of very fine timings that produce certain colors. I don't think that these can possibly affect color with individual, solitary held notes----there must be interaction over time for different colors to be produced.