Why does corn turn yellow?

There are two answers to this question, depending on what you mean:

1. If you mean what makes the plants yellow as they're growing, there are several possible causes, most of which are nutrient-related. A deficiency of nitrogen can make the plants yellow. This is somewhat common because there are several causes such as excess rainfall washing nitrogen out of the soil or cool temperatures not allowing ammonium forms of nitrogen to "mineralize" into a form usable by the corn plant. Other nutrient deficiencies can also cause yellowing.
2. If you mean what makes the kernels yellow, corn gets its color from the xanthophyll pigments lutein and zeaxanthine plus the vitamin A-active pigments carotene and cryptoxanthin.