It takes around a half hour for an individual to change color. They do so by controlling the pigment in their star-shaped skin cells. Though they can only go from green to gray and back again, they can also control the intensity of the dark splotch pattern found on the back. The sides appear to stay gray for the most part regardless of the chosen back color. Against natural settings, Gray Tree Frogs are masters of camouflage. Since the color choice is primarily intended for the daytime rest period (they are nocturnal) Gray Tree Frogs can pass the daylight hours in either color mode depending on background. In the photo below, this fellow was resting up against the chunk of bark and his pattern matched perfectly. The second photo is of the same frog at night, at which time he was in green mode...For the photos mentioned in the excerpt, see the original post.
Photo credit to Gerry Wykes at Naturespeak.