In some species, there are vastly different color variations between the male and the female such as in the House Finches. During mating season the male becomes bright red in the head and breast area. The female is tan and white.
Sometimes differences can be subtle such as in the Red-bellied WP. The slight difference in the extension of the red on the head.
In the case of Eastern Bluebirds, both are blue with rufous colored breast feathers and white belly feathers. The female is much more subdued particularly the blue on the head.
The male and female of the Eastern Towhee have similar marking appearances with the exception that the upper part of the body of the female is brown versus the black of the male.