Two-Tone Layering Question

so if I just barely hit the roof part with purple, like a little splotch over the line i want to make, and not spray the whole roof purple, I should be ok with a gallon? I was hoping to do the door jams yes, but not the upper door trim on the interior. That's already done, painted black.

Basically the whole roof black, except the front A-pillars, and the stripe black, window frames black, and rear of trunk black between the tail lights.