" Instrumentation" is the best term that can be found for that aspect of musical art which is concerned with timbre.
The narrower term "orchestration" is applied to the instrumentation of orchestral music. Since the most obvious differences of timbre are in those of various instruments, the art which blends and contrasts timbre is most easily discussed as the treatment of instruments; but we must use this term with philosophic breadth and allow it to include voices.
Instrumentation is in all standard text-books treated as a technical subject, from the point of view of practical students desirous of writing for the modern orchestra.
Throughout the 19th century so fatal was the hold obtained on the popular mind by the technical expert's view of instrumentation, that it was impossible to hear the works of Handel and Bach without "additional accompaniments" conceived in terms of art as irrelevant to those of 18th-century polyphonys as the terms of Turnerian landscape are irrelevant to the decoration of the outside walls of a cathedral.
There is some reason to hope that the day of these misconceptions is passed; although there is also some reason to fear that on other grounds the present era may be known to posterity as an era of instrumentation comparable, in its gorgeous chaos of experiment and its lack of consistent ideas of harmony and form, only to the monodic period at the beginning of the 17th century, in which no one had ears for anything but experiments in harmonic colour.