It should be in the FAQ. The colors follow the rainbow. Each power of 10 of sats moves you to the next color.

no, it's not new.

grey: posts you didn't zap green: you zapped, but not much blue: you zapped and it's a good amount ( form my experience, zaps >1k ) a deeper blue: if you zap twice 👀

I think it's been there for a while. It has to do with amounts zapped, I think. I usually am lazy and repeatedly zap a post that I like, rather than holding it down for a custom amount.

Each color represent the amount of sats you've zapped to that post/comment.

That's the amount of love you put in a zap. 😘