Advice/opinions; hair dye/undercut

I ordered curlformers and I'm really excited for them to get here.

I also want to (possibly) dye my hair and get a triangle undercut.

Has anyone shaved an undercut for themselves before?

I've never done more than trim my hair so I'm worried I'll mess it up (being that I wouldn't be able to see the back of my head too well) and my bf refuses cause he's Sure he'll mess it up and doesn't want to be responsible. I don't even care if it's not perfect, I'm just nervous.

Should I try wait until I can get a professional haircut, just for a quick shave? Find a willing acquaintance and hope I can trust them with a simple task? Or trust myself and just go for it?

Also if I dye my hair I want to do a split dye, and red and purple both hold really well in my hair and fade nicely so upkeep is simple. Would those two colors go well together? Should I do black on one side instead?

How would I color the shave??

One solid separate color? Or switch the side the colors are on? Or one of the solid colors and dots or something with the other color?

Please leave advice