No worries! It's such a good question.
This is what I posted in the FAQ. :) I copied and pasted!!
Why does Kate (Lauren Koslow) paint two fingernails black?
While we do not know for certain why Kate does this, we have a few thoughts from people concerning this.
Here they are:
Some say that she paints them black as a tribute to David Bowie, as she is a fan! Others think that it is a fashion statement for her character.
If we ever get the opportunity to interview her, we will ask her, and post it on our site!.