How to create a rainbow

Hi community! 

I'm new to photoshop and made this experiment a few weeks ago. I made this rainbow circle and a rainbow streak using a tool, but I can't remember how I did it, and the history has since been wiped! It has also been merged into one image so I can't isolate layers! Very annoying as I was learning and now can't remember how I created these rainbow effects. Does anybody have any ideas? I remember it being a tool I think...