How to create a rainbow

[Claudia6C14]

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... 

[Chuck Uebele]

You can try this tutorial: https://www.youtube.com/watch?v=LHwOIMbCBCI

[c.pfaffenbichler]

Shape Burst Stroke Layer Style on a Layer with Fill 0% seems like a possibility. 

To »soften« it one could convert it to a Smart Object and apply Gaussian Blur and reduce Opacity.