Do you mean the preview area where Simon was, or the whole workspace screen?
If it's the whole screen, try clicking on the little paintpot icon in the tools bar and clicking the screen. You should be able to choose the colour you want from the colour picker. If it's the preview window, at the bottom right of the preview screen is a set of four little circles. Click the second one (it should say Background Color at the top of the preview window) and again you should be able to set the colour you want.