Paint marker, available at Home Depot, Wal-Mart, etc. Redid the red dots on my M9 a couple weeks ago. I've fired and cleaned it since then with no problems, and paints still there. For detail areas, such as the edge, use a wire keep the color in the lines.
+1 Wally World has it, Hobby Lobby, etc....BTW, I used a rd-nose toothpick for the slight drop...Just go buy model paint - Testors. That's what everyone typically uses - especially to repaint white dots on a sight.
If there is a depression where the dot goes - use a needle and dip it in the paint.
Then, let a drop or two drip off the needle. Then, put it over the spot you want. Get 1 drop to fall where it needs to. It usually will fill the depression perfectly.
I think I would try red Loctite. Clean the spot with acetone, dry and apply Loctite, we know it sticks to metal and you can use the rest for other jobs.
Good luck.
Loctite will never dry in air. It is chemically made as an anaerobic cure, takes a confined space with no air to cure/dry.
Testor's Model Paint is the answer. It has been for years, in fact.