How do i hide my birthmark ? products ?

I personally wouldn't even worry about a birth mark on my knee, but I completely understand where you are coming from. There is a makeup brand called Dermablend and they make makeup for your body, you can check out their website and see before and after pics. I think they sell the stuff at Macy's so next time you are there you can have a sales ociate help you out and find a color that matches your skin tone.