Ive used both Testors and nail polish, with good results. The nail polish usually lasts a lot longer, especially on sights that arent protected and subject to normal wear and tear. The nail polish usually isnt bothers by most gun cleaning solvents, wich will readily remove Testors.
As was mentioned, using a gloss white as a base, dramatically improves the vividness of color you want. This is especially noticeable with the florescents.
Another alternative for the AK's, is a tritium front sight from Meprolight. It offers a larger, sharper front blade, with a tritium lit "dot" that can be seen in total darkness. I have these on all my AK's, and they work very well, especially for quick shooting. The only downside is, you reduce your working range by about 100 yards (they are good out to 200 yards or so) for any kind of precision shooting. You can still make decent COM hits at 300 yards, but the blade is to "fat" at that distance, and usually completely covers an IPSC type target, making it a bit more difficult if youre into shooting "groups".