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.