The Somewhat Amusing World of Frogs

Frogs range across very many colours (even bright blue):
(Mattison)
The blue frog above is a poison arrow frog. These frogs are found in Central and South America and, as their name suggests, may be used to produce poison arrows. The frogs secrete toxins from their skin which can kill predators. They are brightly coloured to warn predators to stay away.
However, most frogs are dull in colour (to blend in with their environment) because frogs generally mate at night (so no-one sees colour) and croaking is the important discriminator (the louder the better).
Some species of frogs are "polymorphic" - ie. there may be different colourings in the same population (eg. speckled, striped and plain) - this makes it difficult for predators to get their eye in (they don't know what to look for).
Frogs often have skin colouration that helps them blend into their environment and hide from predators:
(Hess)
but that may not help a frog much in autumn:
(Hess)
Male and female frogs tend to be the same colour. However, there are often differences in markings/skin that may indicate whether a frog is male or female. Other factors used to determine a frog's sex include the fact that males call (females don't) and the size of a frog. The difficulty in determining a frog's sex is due to frogs generally not having external genitalia.
What's green green green green green? A frog rolling down a hill