Translation:An eggplant is not a purple banana.
Why does the adjective not follow the rule that colours derived from objects (orange etc) do not decline ?
The color adjective agreement rule has exceptions which also have exceptions: "violet, violette, violets, violettes" is one of them.
I will be honest: I have never heard of an aubergine, and I am a full-blooded American
It's because you are a full-blooded American that you have never heard of it; eggplant is the American term for aubergine.
In AmE, it is an eggplant. In the UK, it's called an aubergine (borrowed from the French). It's not everyone's cup of tea.
This is another case of duolingo preferring a non-obvious translation. The obvious translation for aubergine is aubergine since we use the same word in English yet Duo prefers the more obscure "egg plant".
"Aubergine" is also accepted but not the preferred translation in American English.
In the U.K. it may be obvious to say "aubergine" which is a word you borrowed from the French. In the States, very few people would know that and use the common word "eggplant" (not "egg plant"). Nevertheless, both "eggplant" and "aubergine" are accepted so there is no real problem.