On a dark background a frog will darken in color, on a light background they brighten in color. So if your enclosure has a black background, like mine does, and your frog hangs out around the back of the tank, he/she will be brown. If the enclosure is light in color, say like white or yellow, the frog should turn green.
Frogs will also lighten and darken depending on the amount of light they are receiving. My red-eyes are a bright-leaf green during the day. After dark they turn dark green.
And that, my friends is the skinny on the changing of color in frogs (most of the time).