I spent today pollinating maize. My reward was an armful of sweet corn! (currently boiling...)
Like the picture to the right, today's ears contain a mix of white and yellow kernels.
The yellow of maize kernels (along with most yellows, oranges and reds of other fruit) is caused by the accumulation of carotenoids. This chemical class has often been associated with human health and includes such famous members as lycopene and beta-carotene.
One particular enzyme, phytoene synthase, plays a particularly important role in the accumulation of carotenoids. Maize has two different versions of this gene - one for synthesizing carotenoids in the kernel endosperm, and the other for synthesizing it everywhere else. White kernels lack a functional copy of the former phytoene synthase gene, and therefore aren't able to accumulate (yellow) carotenoids.
Ultimately, the color is determined by the combination of the mother plant's genotype in addition to the genotype of the individual pollen grain that fertilized each kernel.