1. Leaves turn gold in autumn.
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Can we use 'gold' in this position? What other words can we use instead?
Yes, "gold" in this sentence is an adjective, in the position of a predicate adjective because the verb "turn" is something of a linking verb.
http://grammar.ccc.commnet.edu/grammar/verbs.htm#linking
Other adjectives that would fit in this particular sentence are other color adjectives:
yellow
orange
red
brown