I'm with GM---decals would be a much simpler way to go.
Years (decades) ago I hand-painted lozenges on the wings of a Revell 1/72 Fokker D.VII. I had a sharp eye and a steady hand, and the results weren't awful---I laid out the pattern in pencil, then 'filled in' the colors---but even with a steady hand, the problem ultimately was making the pattern look uniform enough. It's nearly impossible to keep the layout even enough to look anything but...well, hand-painted.
Just my two Deutschmarks. Good luck.