Set a Background Colour in TextEdit
In TextEdit, go to the Format menu and choose Show Fonts. In the Font palette that appears, you will notice four buttons in the top left - 3 with a T on them and one with a small page icon. If you can't see these icons, your font palette might be too small. Drag the handle in the bottom right to resize it.
The button we want is the fourth one along. It should be a small page icon with a white box next to it. Clicking it brings up the colors palette which you can use to select a new background colour.
You are best off using the colour pickers that allow you to drag around to select a nice subtle colour. The preset colours (like in the crayon picker) are far too bright for a background.
Also note that any background colour that you set will be printed, so you will have to change it back to white before printing to avoid wasting ink.