Could you give me an example? I'd love to know more about the watercolour paint mixing. If you meant the "live tip" settings of Photoshop, you can do all that in Krita too, using the "texture", and "mask tip" features. RGBA seems to definitely be something Krita has over Photoshop, but I could well be wrong!

by the way, there are other softwares like Rebelle that try to truly simulate traditional mediums - bordering on a whole-ass physics engine that works completely different in the backend from PSD/Krita. Unfortunately its a paid software so yeah :s