Ehehehe...with the massive swatch list I almost didn't recognize Verve, hehe!
Well, it's easy to explain what is happening for you there, but the reason for why it has to be that way is not as easy, maybe:
- When you choose a color, all pixels with an alpha value below a certain threshold on the layer will get filled with that color, not changing the alpha value. However, when fluids smear across pixels that had just enough alpha not to be flushed with the new color, they will smear what ever old color was still on them. It's a weird thing, but it's a compromise I went for in order to make things act as fast as they do.
NORMALLY one would not really paint on the layer during painting, but on a temporary extra layer. If I did that with Verve, I would have to apply fluids to both layers and then do the even harder process of mixing existing colors with the temporary new layer and so on... a big, slow pain to deal with.
Anyway, with how I have solved it, there are some fun side effects one can actually exploit nicely, too!