Chicago Herpetological Society

Gray Treefrog Complex Hyla versicolor Hyla chrysoscelis Hyla versicolor-chrysoscelis	Gray Treefrog (Hyla versicolor)
Hyla versicolor, the Gray Treefrog, and Hyla chrysoscelis, Cope's Gray Treefrog, are similar in appearance. In general, the Cope's Gray Treefog tends to be smaller and have smoother skin than the Gray Treefrog, and it is often bright green when calling. Nevertheless these character traits overlap, making them unreliable for identification. Their calls are different as well, however you would have to be quite an expert to be confident about the identification. Another difference is that the Cope's Gray Treefrog has a diploid complement of chromosomes and the Gray Treefrog is tetraploid so blood tests and chromosome analysis can distinguish the two although this technique is not available to the average person.
Illinois Natural History Survey Range Map: Hyla versicolor Hyla chrysoscelis Hyla versicolor-chrysoscelis	Cope's Gray Treefrog (Hyla chrysoscelis)

Gray Treefrog showing the bright flash color normally hidden on the hind legs. This color is used to startle and confuse predators. A predator sees bright yellow hopping. When the frog stops and tucks in his legs the bright color is instantly gone.

The following following four photographs were taken of a single individual frog over a 24 hour period, and show the remarkable degree of color change they can undergo The frog gradually changes color from gray to green.