Hi Marieke, I think I need to know a little bit more before I try to answer. The area that is circled you want to be completely white?
As far as I know, the color bleed-over from the beige wall on the right is an effect that would take place in the real world in that physical situation, so it looks like that's what is happening. If you're willing to post a Maxwell Model to "Pack'n'Go" then I'd be happy to take a look!
Matt