Hourly staff in my store can wear whatever they want as long as it has a closed toe and heel. Color is irrelevant. You're alright as long as it's closed toe. You can't go into Walmart and buy slippers but they will accept your old ones if you like the look of them. The reason why they don't sell them is because people use them as an opportunity to not walk around their store but drive instead which isn't good for your feet or the environment. If you ask me, they should allow this practice in the store.