Timbre or tone color — the characteristic quality of a musical sound that distinguishes different instruments or voices.
ピアノの音色がとても美しい。
The piano's tone color is very beautiful.
ギターは弦の種類で音色が変わる。
A guitar's timbre changes depending on the type of strings.
同じメロディーでも楽器が変わると音色が違って聞こえる。
The same melody sounds different in timbre when played on different instruments.