White quinoa seeds, the largest of the three varieties, have a slightly nutty, vegetal flavor with a hint of bitterness; they also have the softest texture of the three quinoas. The medium-size red seeds offer a heartier crunch, thanks to their additional seed coat and a predominant nuttiness. Black quinoa seeds, the smallest of the three, have the thickest seed coat. They were notably crunchy and retained their shape the most during cooking, but many tasters disliked their slightly sandy texture. These seeds have the mildest flavor, with a hint of molasses-like sweetness.
(Source: Ammerica's Test Kitchen)