There was a word, 木色【きいろ】 "the colour of trees", recorded in the Vocabulário da Língua do Japão. But actually, 黄【き】 on its own already means "yellow".
木 and 黄 are most likely not etymologically related. We know that 木 had a type-2 (乙類) /ki/ in Old Japanese. If we knew that 黄 had a type-1 (甲類) /ki/, then we could definitively say that the two are etymologically unrelated. This diagnostic is the reason why we say that 神【かみ】 and 上【かみ】 are unrelated words: the former has a type-2 /mi/, while the latter has a type-1 /mi/. However, we do know the modern accentuation patterns of 木 and 黄, and they are different:
- 木 is accented in modern standard Japanese, 黄 is not.
- 木【きい】 is low register unaccented (→ LH) in modern Kyoto dialect, 黄【きい】 is high register unaccented (→ HH).
Assuming these continue a historical distinction, it seems safe to conclude that 木 and 黄 were not the same word in Old Japanese as well.