Movie Poster colourful background question

I'm trying to make a poster for Avengers: Infinity War, and I saw this really cool design that I wanted to try creating (in my own way). However, I can't seem to figure out what he used to get all of those colorful layers and what they're called when you search them up on google. I tried using a few different things as my background for the characters, but it's not looking as professional as I want it to be. Any help would be great.