谷は赤や黄色の葉で色づいていた。

valley

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

red; crimson; scarlet; red-containing colour (e.g. brown, pink, orange); Red (i.e. communist); red light; red ink (i.e. in finance or proof-reading); (in) the red; complete; total; perfect; obvious; copper

such things as (non-exhaustive list related to a specific time and place); and ... and; the minute (that) ...; no sooner than ...; as soon as; punctuational exclamation in haiku, renga, etc.; interjection expressing surprise; hi; plain copula; yes; what?; indicates a question

yellow; amber

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

leaf

indicates location of action; at; in; indicates time of action; indicates means of action; cause of effect; by; and then; so; indicates continuing action; indicates certainty, emphasis, etc.

to change color (esp. leaves, fruit, flowers) (colour); to turn crimson (e.g. leaves in autumn); to begin to redden; to ripen; to grow ripe