HOW do you remove a texture?!?

There is a trick, that is, to say the least, totally unprofessional and everything else but clean :o)

Unfortunately you cannot use zero as a texture handle to remove a texture from a brush (eg. BrushTexture mybrush,0 )
BUT you can use an invalid texture handle:

let's assume tex1 is an existing texture

BrushTexture brush,tex1
PaintMesh mesh,brush

Now it's textured as usual. Now here comes the dirty bit:

FreeTexture tex1
BrushTexture brush,tex1
PaintMesh mesh,brush

Et voila, the texture is removed. Tho, I have no idea if this works on all machines, or what troubles it's gonna cause.

BTW instead of PaintMesh you maybe better use PaintSurface.