These frogs have the ability to change color from brown to green.
Previously, adults were thought to occur in two different fixed colors.
Now, some of them have been found to be able to change between the two.
They can also change from lighter to darker, shift from patterned markings to pure colors and vice versa, and even display combinations of colors, brown/green being the most frequent.

These color-changing morphs are triggered not by color change in their environments, but a change in background brightness.
This type of environmental change would be caused by seasonal fluctuation.
A full change in the dorsal coloration of a color morph can take from weeks to months, but initial changes can occur in just a few hours.
This has been shown to be a very useful cryptic survival feature for these frogs.

Skin color is produced via pigment cells called chromatophores.
 Three types of chromatophores are commonly found in amphibians:  Xanthophores, which contain yellow, orange, or red pigments and are found uppermost on the dermis;  iridophores, which lie below the xanthophores and function by reflecting and scattering white light up through them (in the case of Pseudacris regilla and many other North American frogs, iridophores reflect blue light through the yellow pigment cells above to create a green frog) and melanophores, which are the deepest pigment cells and are responsible for the presence of black and brown pigments.

A rare, recessive "blue morph" is known.
 One is housed at the Humboldt State University Natural History Museum in Arcata, California.
 This mutation might inhibit the xanthophores' ability to produce yellow pigments, thus the normally green frog (possibly of the color-unchanging type) appears blue.
These frogs have the ability to change color from brown to green. Previously, adults were thought to occur in two different fixed colors. Now, some of them have been found to be able to change between the two. They can also change from lighter to darker, shift from patterned markings to pure colors and vice versa, and even display combinations of colors, brown/green being the most frequent.
These color-changing morphs are triggered not by color change in their environments, but a change in background brightness. This type of environmental change would be caused by seasonal fluctuation. A full change in the dorsal coloration of a color morph can take from weeks to months, but initial changes can occur in just a few hours. This has been shown to be a very useful cryptic survival feature for these frogs.
Skin color is produced via pigment cells called chromatophores.  Three types of chromatophores are commonly found in amphibians:  Xanthophores, which contain yellow, orange, or red pigments and are found uppermost on the dermis;  iridophores, which lie below the xanthophores and function by reflecting and scattering white light up through them (in the case of Pseudacris regilla and many other North American frogs, iridophores reflect blue light through the yellow pigment cells above to create a green frog) and melanophores, which are the deepest pigment cells and are responsible for the presence of black and brown pigments.
A rare, recessive "blue morph" is known.  One is housed at the Humboldt State University Natural History Museum in Arcata, California.  This mutation might inhibit the xanthophores' ability to produce yellow pigments, thus the normally green frog (possibly of the color-unchanging type) appears blue.
Pacific tree frog