I thought green was the colour humans saw best, if best means brightest. For instance, when converting from RGB to greyscale the usual weights are 30% red, 59% green, and 11% blue. (Though I always found it kind of unfortunate that these weights are wrong for people with nonstandard colour receptors.) In that sense, green should (seem to) be the largest because we perceive more colours as greenish than as reddish or bluish.