My T suggested this same thing once. She actually suggested just scribbling on paper, and I just cringed. Nope, I really can not do it. My concession was just dumping the crayons in to a tin without organizing them...a small step, but I'm working on it.

Honestly, I find coloring within the lines very calming, relaxing. It's a nice mindless task that allows me to let go of some anxiety. Trying to actively color "wrong" by coloring outside the lines is stressful and difficult.

So, I understand what you're going through...no advice on how to actually do the coloring outside the lines, though. I simply told my T it wasn't an activity I thought would be useful for me.