Wait… moldy painting color?
I didn’t even know that this could happen at all. There is a spot of mold growing in my scorched brown. Well, the pot is in use for about 7 years now but I thought the worst thing to happen to acrylic colors is drying-out. It is not that good to spot in the picture ( the white part in the corner). Let me tell you that it is of the furry type.
Probably there is some organic contamination going on. At least the pot is already almost empty and just some weeks ago I wanted to buy a new brown but it was sold out at my LGS.