I'm not an expert, but here's how I would do it...(this works best if you have the box and the text on two different layers) Just set the primary color to the color you want for the background, select the paint bucket tool, and click somewhere inside the border. It should fill in with that color. As for the text, I would delete it and re-write it in the color you want. If the text remains selected, you can change its color by clicking another primary color. But after text is deselected, there's really no way to go back and edit it--at least that I know of.
Hope that helps!