I get this with painting or high-spec art when I throw down color directly on the canvas to blend instead of mixing it on the palette because I'm lazy and don't care if my stuff has slightly wrong colors at the edges of the mix or somewhere inbetween. Plus, some people like the rustic or traditional look of it.

Far as I can tell though, it doesn't work very well with thin gouache or watercolor because the weird color for me was the result of thick paint 'pushing colors' and not blending perfectly. But I get the same effect with gouache by just layering wet paint on dry and you also get those bands or spots of slightly-not-the-right-color.

Maybe someone else has another reason for this existing, I assume it's got to do with how ambient light affects material, but in my case it's always been an accident that gives other people the illusion of some deeper understanding of color or whatever.