I found a nice example of a bicolour stamp. The most likely explanation for this is that a part of a stamp was exposed to UV light. However, it is interesting that colours perfectly match known colours of 6pf stamp - so one might believe that this occurred while printed...
1945 Maribor provisional issue, a pair of 6pf with shifted overprint. Right stamp in two colours: light purple lilac (785d) and light pink lilac (785c).
Known colours of 6pf stamp.
