I've seen girls with natural nails, fake nails, painted and unpainted. I don't think there's a rule. All I know is that they told me at my first interview that they expect you to always look your best. So they expect you to always have your nails manicured. If you have fake nails they want them to be and not have you grow them out for a month or two before you go get them fixed again. Or have a missing nail here and there.
They never said anything about nail polish colors but considering they sell all types of colored nail polish I don't think it matters.
I personally am growing out my real nails after having horrible fake nails for about 4 years. My nails are all broken and down to the nob because they're still so weak that they grow and then break. I don't think they're too strick on it though. As long as you don't have gross long and dirty nails I don't think they'll have a problem.