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Aphid's Color Comes From a Fungus Gene
Iron Nose writes with an account from Byte Size Biology of horizontal gene transfer from a fungus to an insect. The author suspects that we will see lots more of this as we sequence more genomes. "The pea aphid is known for having two different colors, green and red, but until now it was not clear how the aphids got their color. Aphids feed on sap, and sap does not contain carotenoids, a common pigment synthesized by plants, fungi, and microbes, but not by animals. Carotenoids in the diet gives many animals, from insects to flamingos, their exterior color after they ingest it, but aphids do not seem to eat carotenoid-containing food. Nancy Moran and Tyler Jarvik from the University of Arizona looked at the recently sequenced genome of the pea aphid. They were surprised to find genes for synthesizing carotenoids; this is the first time carotenoid synthesizing genes have been found in animals. When the researchers looked for the most similar genes to the aphid carotenoid synthesizing genes, they found that they came from fungi, which means they somehow jumped between fungi and aphids, in a process known as horizontal gene transfer."
According to this hottie (see link) black and brown are natural colors produced by pigments; usually red, oranges and yellows are the carotenoids which animals get from foods, and blues and greens (in birds) come from microstructure rather than actual color. (Obviously a green caterpillar gets the color from the diet. A bit different for animals, since I've never seen a green cow.)
http://www.learnoutloud.com/Catalog/Science/Biology/Basics-of-Genetics/31316
She also says that horizontal gene transfer is very common, and that 90% of our DNA is viral. The viruses we hear about are the ones that make us sick. The ones that have no ill effects we don't notice so much; these are also called viruses or jumping genes.
http://wheatoncollege.edu/quarterly/q2003fall/bacteria.html