Chrome

In the print window, click on "More settings", then under "Options", select "Background graphics".

Firefox

In the print window, click on "Page Setup".  Under Options, select "Print Background (colors & images)".

Safari

From the print window, click on "Show Details", and then select "Print backgrounds".

Internet Explorer

Right-click the document and select "Print preview..."  From the preview window, click on the settings (gear) icon.  Under "Paper Options", select "Print Background Colors and Images".