It used to be a thing, but after one of the updates, the colour choice was swapped for different icon shapes (like a heart, a flower, or a dog paw). I appreciate those as well, but I regret not being able to colour-code anymore. A Textra icon with different colours sends a clear message ("you got a text from this specific person"), while shapes not related to texting could mean plenty of things coming from different apps (I think a creator loving your comment on YouTube or somewhere else has a heart-shaped icon as well).