The general consensus is that foreign substances that would significantly alter the performance/function of a Beyblade or part aren't allowed, such applying nail polish to areas of considerable contact with the stadium, other Beys, or parts that connect for assembly. So as long as your dab of nail polish isn't in any places like those, it should be alright, though, a dot of sharpie anywhere would also do the trick.
The rule for paint is similar in that a thin layer that doesn't significantly alter the weight or performance/function of a Beyblade part is okay as well. However, I personally haven't seen many painted Beys used in tournaments, since their paint always seems to chip and make a mess. Dying plastic parts has always been my preferred method of recoloring, but you have to really know what you're doing, or else you can permanently damage the piece.
The rule for paint is similar in that a thin layer that doesn't significantly alter the weight or performance/function of a Beyblade part is okay as well. However, I personally haven't seen many painted Beys used in tournaments, since their paint always seems to chip and make a mess. Dying plastic parts has always been my preferred method of recoloring, but you have to really know what you're doing, or else you can permanently damage the piece.