Krita is absolutely incredible FOSS, really well polished, feels a joy to use, very intuitive. There are a few quirks, such as changing the colour of a vector,

Krita is absolutely incredible FOSS, really well polished, feels a joy to use, very intuitive. There are a few quirks, such as changing the colour of a vector, you have to 'stroke' it (i.e. turn it into a raster) with a particular colour -- the feedback loop for this is fairly painful, but admittedly there's lots about the software I don't know, so there may be a better way.