You can always mist whatever color your trying to regain. But if you take your time with the black or whatever color your misting you shouldn't have to reapply color. Just go really slow or practcce on something else. The only thing you might have to retouch is the silver areas and you can do that with a brush. you shouldn't have to mask anything off unless your visor is already installed.
If your worried about misting have you thought about black washing the helmet. Mix a little black paint with a lot of water, brush it on, use a paper towel or something to wipe of areas that have to much paint or water. It's a pretty easy method. Just use acrylic paint. The mixture of paint and water should be mostly water. I think I read some people use four parts water to one part paint. I just mix as I go along.
You can also grind up pastels or charcoal and brush that on. You can then use a clear spray to get it to stay on. You can also mix water with the ground up pastels or charcoal and it will act like a water color. If you do this practice first because they will stain whatever their going on. If it's too dark a mix your helmet will get nasty stains on it that will have to be repainted.
