Draw inside a character

Hi Guys

This is my first post so I'll apologise in advance

I was wondering if someone could help me with an idea that I have

I am wanting to draw inside a keyline image eg

I have a cap and I want to paint the body green and the peak blue

If I was painting the peak and I happened to run into the body then only the peak would colour and visa versa

Appreciate any help

Bear :sign0163: