Question

Which element is described by words such as bright, dark, and mellow?

Answer 1

Your answer is Timbre

Which is defined as sonorous quality creating various expressive effects, described with words such as bright, dark, brilliant, mellow and rich.

Hope this helps.

Answer 2

Timbre 
defined as sonorous quality creating various expressive effects, described with words such as bright, dark, brilliant, mellow and rich.