I used to do that too, Erica!
I had this beautiful colouring book just sitting there because I didn't want to stuff up any of the illustrations. I finally ended up just going with it and forcing myself not to think about what colour would look best and what would match the rest. I pick a totally different colour for each little part and it becomes some kind of fabulous rainbow – not perfect and not like how it's probably 'supposed' to look but I'm getting better at just going with it.
I've also been sketching by copying from a drawing book (things that my daughter can then colour in) and I've completed some paintings on canvas (none of it even close to perfect). I was so afraid of painting on canvas until I told myself that I can paint over them and turn them into something else. I can't even tell you how much that helped – just telling myself that it's okay because I can repurpose and reuse.
And that ended up really long. Sorry about that. I'm still surprised at how far I've come with giving these things a go that I spent so long feeling afraid to even attempt (not that I don't have days when I'm still afraid but I've made good progress).