Natural rubber goes through a process called oxidation when left open and/or in highly humid conditions. Hence, when you wash the boots, you see a white layer. The colour has not faded. The boots will return to their original colour once dry. However, if boots are left in open for a very long time, the oxidation may become permanent. If you plan to not use them for a long period, dust the boots with talcum powder and store them in a dark box.