I painted my SP101 sight with a base coat of white Testors enamal and then used a fluorescent lime green acrylic paint for the top coat. I like the lime green better than the orange I used to use. I bought my paints at Hobby Lobby.
A round toothpick is the best thing I have found to paint the sights with.
Another vote for Nail Polish. I found some that are UV sensitive at the Dollar Store. Pick any color that works for you. White makes a good base coat, but is not good for a top coat if you think you will be doing any shooting out in the snow. Same with steel gong targets; white works great, until it snows.
I used a two step process.
First I used the tub enamel chip repair paint. It is SUPER WHITE in color, sticks to anything and is super hard when dry.
It also gives a very nice base for what ever paint you want to use for the front sight. Also you do not have to worry about chemicals cleaning or paint remover should you want to change sight color.
Then after trying a few different nail polishes settled on a bright orange.
Then you file a section out of the sight using a triangular file so it's undercut, put some plastic temporary "dams" on either side, fill the cut with a colored epoxy and then file the resulting inset to shape.
That's a nice permanent solution although it does require a good bit more work than paint.