Yes, on the subject of harmonics, I agree with Binko_Binobo. It is absolutely true that as the note get louder, (with greater force f=ma, changes in velocity is the acceleration), the more higher harmonics appear, and their numbers also increase. So science is right in the respect that one single note, no matter how you play it with different touches, as long as the velocity is still the same, the note will sound exactly the same.
However, if we are talking about a few more notes, such as a linear melody, there are certain things performer does that differs from other performers. We know that sound is produced through 4 major acoustic parameters, the frquency of the sound, which determines pitch, the Amplitude of sound which determines Dynmanics, (these 2 are very closely related however,) the envelope of the sound, which determines the attack, Deccay, Sustain and Release of the sound (Duration,) and the Spectrum of sound, which determines the Timbre (instrument wise.) (I learn something new in uni every day.) Combining these factors together in a single note at a time melody, there's no performance that is ever the same. That is what determines the overall "tone colour" and touch of the piano and its operator.