Krita doesn’t have neither RYB color wheel, nor pigment mixing (which you’d need too, if you expect blue and yellow to create green). The RYB color wheel might be in Pigment.O plugin: Pigment.O plugin but I don’t know for sure.
The pigment mixing is implemented in an open source and free program called MyPaint, you can check it out. It’s much simpler than Krita but it has its advantages.
If/when the libmypaint library which we use for MyPaint brushes include pigment mixing, we might get it in Krita too. But for now, it’s not there yet.