I think anything that will take out the nail polish (nail polish remover, rubbing alcohol, etc.) will also probably take the color out of the leather. So, its probably a question of what is worse to you - nail polish on your bag or a light spot on the leather.
I had a similar situation a while ago when I got ink on my Balenciaga. I used a little alcohol on a q-tip and it took the ink out, but it also made the leather lighter (which was ok with me, since it was a lighter color leather anyway).
Good luck!