I have used white model paint on a few guns in the past - never a problem.
I painted the front site of my Keltec 32 white - Actually looks very good. I was amazed it came out so well - almost factory looking.
My USPc site has a white dot that came off - HK uses this white plastic that they glue on. It is common that these come off. I painted the depressed dot - looks perfect.
Also, I had a 1911 years ago with a comp - the front site always got dirty after a few trips to the range.