Quick grammar question: not superlative but on its way?

Hi!
“Purple” is an adjective. “Purplest” is the adjective in its superlative form. What’s the fancy way of describing “purpler” then? Is there a term for it? Like, “progressive form,” maybe? Thaaaaanks.

P.S. The adjective I’ll be “progressing” is “twatty,” but I didn’t want to have to mark this NSFW ;)