Final answer:
An organist changes the timbre of sound using tone color or timbre, which is the characteristic that distinguishes different sounds of the same pitch and volume.
Step-by-step explanation:
An organist uses tone color, also known as timbre, to change the timbre of the sound. Timbre refers to the character or quality of a musical note or sound that distinguishes it from other sounds at the same pitch and volume. The perception of timbre is a complex phenomenon that involves the listener's response to the frequency, amplitude, and timing of sound waves. When an organist wishes to adjust the timbre, they may do so by selecting different combinations of organ stops, which changes the mixture of overtones heard and alters the resulting sound.