灰(huī) - grey in Mandarin

灰 (huī) means grey, and on its own, without a colour sense at all, it also means ash or dust, the fine grey residue left after burning. The colour sense grew directly out of that literal one.

As with the other colour words, 灰色 (huīsè), "grey colour", is the form you hear most before a noun: 灰色的夹克 (huīsè de jiākè), a grey jacket. As a predicate it takes 是...的 (shì...de) or a degree adverb: 这条皮带是灰的 (zhè tiáo pídài shì huī de), this belt is grey.

灰 (huī) also carries a figurative sense of discouragement or defeat, as in 灰心 (huīxīn), literally "ashen heart", meaning to lose heart or become disheartened. That is a different word to learn later, but worth flagging now so the character rings a bell when you meet it.