Hi Varg!

In fact I already noticed your post two days ago and tested the vary colour slider out to find an answer. But that doesn't seem to be so easy. At the moment I think, that it works somehow similar to the Sticker Variation (Sticker Spray Tool) Random/Hue. This means that the brush paints a sequence of different colours, depending on how much you drag the slider to the right. At first it paints only one colour (0), then a sequence of two colours related to the colour wheel (f.e. from red to yellow). And so on. But I'm not really sure.