Doug,
Thanks again for the information of food influencing color.I hadn't realized that previously!
Interestingly, some birds that we think are drab or are similar in color, can readily be differentiated under UV light and a looking through a camera with appropriate filters.
Also there are colors in the visible spectrum that birds can discriminate far more easily than we are able to do. There's a great Scientific American article
here that is worth reading.
"They even have colors in their plumage that are invisible to the human eye. Birds have four color cones in their eyes (compared to three in humans), which allow them to see the ultraviolet part of the color spectrum. Scientists using spectroradiometers to measure the extent of ultraviolet coloration have found that males in many apparently monochromatic species (those with similarly colored sexes, such as European starlings) in fact sport bright ultraviolet colors that females use extensively in their choice of mate."
Yellows and greens seem too be more related to the birds genetics but reds are more related to diet. In general, it appears that while the colors of the bird that does the food gathering of the pair, (most often the male), is the most brightly colored. Nevertheless, the main driver of bright colors and chromatic dimorphism is not sexual selection but evolutionary pressures of surviving predators and signally to other males that this territory is occupied by a healthy pair that will oppose you settling here!
Asher