Also in English we avoid repeating the same noun. So I think 'Black and purple umbrellas' should also be accepted.
To improve style, I agree avoiding duplications is good. However, the sentence given has a duplication, so the correct translation does the same. 'Black and purple umbrellas' would be 'payung-payung hitam dan ungu' I think, slightly different, while identical in meaning.
Humm... It might not be identical in meaning. Payung hitam dan payung ungu' describes several unicolour umbrellas. Payung-payung hitam dan ungu can also describe several umbrellas which may have a nice striped pattern of purple and black, yet all look the same.
Could you say 'payung hitam dan ungu'? Or would that mean multiple umbrellas that are all both black and purple?