The colour amaranthine came from the Greek word ‘amarantos’ which meant unfading. The word Amaranth was used to name an imaginary, undying flower that was, presumably, a deep red-purple colour and there you have the two uses of the word today. Lovely stuff.
English is an expansive language, anyone who has seen the size of an unabridged English dictionary can tell you that. It's also a language that's ever changing and evolving, so of course there are some words that fall out of favor and are hardly ever used, but maybe it's time to bring some of them back!