There's even a quiz to give after the session. Browns contain Carbon, are usually dry and typically will burn. Without an energy source these small creatures, typically bacteria, don't have any food and can't grow. Cow manure for example is at first wet and smelly, characteristic of the greens group. Over time, however, it dries and looks brown and is burned for fuel in the Third World.
Leaves make a similar transition over their lifetime.
They are rated by their suitability for making compost. They are concentrated browns requiring a lot of greens and typically a long time to break down. Greens contain Nitrogen, are usually wet and need to be handled quickly as much of their evaporating nitrogen often escapes as ammonia giving the typical bad smell. Like our bodies, a compost pile needs carbo's and protein, and for the same reasons. Blue stands for water.