Hi Kimberlee! Sorry to hear yours turn purple too but, I did recently find out what causes it. Apparently it's from the blood pooling (standing in one spot) & it's suppose to be pretty normal unless it lasts for long durations after the foot is propped up. I was mostly seeing it after standing in the shower for 10+ minutes in one spot.
Mine are doing it less & less as I gradually increase my time on my feet.
Plus massaging the feet a couple of times a day helps to stimulate them & keep the blood flowing - which seems to help them from getting purple when standing I do actually stand on them.
Hope that helps!