Very well, so the observer can still distinguish between the color of the left box and the color of the right box after you have removed the wall separating them. I still assume he can only see the solid color of each box - he sees the entire left box as one color, and the entire right box as one color too. In that case, I predict he will likely see a never ending fluctuation of monochromatic colors. It is possible but highly unlikely that he will see a solid color in either box, unchanging over time. More probably, the color of each box will usually be a middle shade of grey, constantly flickering from lighter to darker tones. Eventually both boxes will return to their original black and white, if only for a moment.