Problem with applying paint or wallpaper to part of a wall

Hello,

I want to decorate half of a wall diffrently to the other half. I have read that I need to enter the "costimize wall" mode. I do that and draw a simple rectangle where I want different colour to be, but I can't aplly any colour or wallpaper to this new rectangle. Nothing happens when I select a wall finish and click on that part of the wall. Please help.