他用低沉的语调告诉我这个秘密。

He told me the secret in a low ___undertone.

他的玩笑话里带着一丝讽刺的意味。

There was an ___undertone of sarcasm in his joke.

这件灰色衬衫在阳光下透出淡淡的绿色底调。

The gray shirt has a slight green ___undertone in the sunlight.