I'm trying something supposedly very simple which is to fill a texture with a color and I seem to get a weird result:
What am I missing here?
Umm not sure if this is a bug of blender. Because I had the same issue too, so a workaround is using the uv image editor and painting your texture on the image editor directly, avoid painting throught the 3d viewport blender doesnt like it, dont know why. :)
To enable painting on the image editor only change the tab that says view to paint.