First I'm going to preach a bit. I feel you should know your pistol well enough that you don't need a color piece to know if it is safe or not.
Second, My thought would be red finger nail polish to remark where the original fell off. With that said I'm not sure specifically what you are referring to so paint might not be the answer.
Welcome to the site. I'm an old P-nut. P-series pistols are my thing.