When I was playing UNO earlier, I found two strange cards: they were purple and blank, with no number or special sign on them. What is this card for? Is it like a Wild Card or +4 Wild Card? It wasn't mentioned in the instructions.
These cards are mentioned in Uno instructions - you can use if you loose or damage one of the other cards, or you can use it to include custom house rules card into the deck. See reference in manual:
It says also that you should have 4 of these, maybe you have lost the other two?