Amaranthine is an adjective that means never fading or undying. 

Our word of the day comes from a plant called an amaranth that is known for never fading. Its name comes from the Greek word amarantos (ah mar RAHN tose) which means ‘not fading.’ Amaranthine is an adjective that can be used to describe any object that doesn’t fade. Example:

Most of my clothes tend to lose their lustre after a few years, but not my amaranthine Grateful Dead t-shirt. It remains as bright and vibrant as I bought it twenty-two years ago.