By naturally, I mean, through the course of natural selection. One-color ears are unnatural products of human selection. Livestock feeders prefer vitamin-rich yellow kernels, Southerners like white kernels, and Native Americans favor blue. Years of deliberate selection, careful pollination, and storing of seeds produced these single-color corn ears. In general, colors help a plant attract or repel other organisms or hide from predators.
Colors also occur as an integral part of biochemical reactions.
For example, the chlorophyll in green leaves reflects green light as it absorbs red and blue light from the Sun during photosynthesis. Some studies suggest corn pigments promote resistance to insects or fungi that invade an ear of corn. By the way, corn has been around a long time. Primitive peoples of Central America or Mexico were the horticultural geniuses who first cultivated corn at least 5,600 years ago.