You can change the note's background colour while you're creating or editing a note, by clicking the icon:


You can also change the note's background colour while the note is displayed on the board, by clicking the toolbar icon:


The toolbar icon has one additional option, "Custom...", which is not available when creating or editing a note. This option lets you choose a picture for the note's background. If you choose a background picture, it will tile to fit the size of the note (ie. the background image will not stretch to fit, but will repeat if the note is larger than the picture, and will crop if the note is smaller than the picture).