We hear these by the thousands, starting up their peeping like little birds, at sunset. Whole choruses of peepers, especially in the spring, but continuing sporadically throughout the summer. I found this one on our stainless steel grill and moved it to a house window. I wonder if it had taken on the color of the grill.
Notice the yellow colors on the underside. Do tree frogs change colors like chameleons? I don't know.
Postscript: - now that I've done a little research, I see this is the Eastern Gray Tree Frog or Common Gray Tree Frog (Hyla versicolor), and they can change color.