Ducks - why are all the females brown?

 Ducks, like gulls and little brown birds, will be a source of frustration for me during my Big Year.  It's easy to find and count males during breeding season when they are sporting their full colors, but the females and immature birds all seem to look brown. There are even some ducks that are brown all year.  Distinctions are between bill color and shape, feather patterns, leg color (which is often hidden because they are on the water), and hidden patches of color under the wings (only seen when they spread their wings to stretch or fly). I believe there are around 50 species of "ducks" that can be found in the continental USA - that includes natives and migrants.