For the past several days I’ve seen a handful of green tree frogs on the horsetail growing in and around the Troodon Exhibit on our Dinosaur Trail. At least one of the frogs is brown. It’s not unusual to find dark green or even brown green tree frogs, although they’re typically bright green.
Over the years, I’ve noticed that the dark green coloration is often associated with cold temperatures. Any green tree frogs I seen very late in the season (October-December), and there aren’t many, are dark green. I’m not sure of the mechanism behind the brown coloration. It could be substrate triggered, the color of whatever the frog was sitting on at the time.
Though the variation in color of the green tree frogs seen here is normal, I’ve also seen a very abnormal “blue” green tree frog at the museum. It’s color was caused by a genetic abnormality in which yellow pigment in the skin is not produced. This condition is referred to as axanthic. For a more thorough look at this abnormality please see this post from 2015: http://www.natureblog.org/blue-green-tree-frog/
Happy frog watching.