I think for a part you are correct, and for a part you are not correct, but if fuxia exist as a way to refer to the color in many places and/or instances, then Fuxia exists as a way to refer to the color and the fact that it does not exist is just theoretical https://www.google.com/search?q=fuxia&tbm=isch

True, but then again Italian has regular pronunciation rules and "x" is always pronounced the same way as "cs", so even if "fuxia" is not a proper spelling it's a homophone and I wouldn't be surprised to see somebody make that spelling mistake.