In UIColor, green is around the 0.4 mark in terms of hue, with red at 0 or 1, and on a standard color wheel, green is opposite red. There's other examples too. How come it's different in UIColor?
There are so many different color spaces -- I think it'd be hard to call one of them "standard" without providing a little more context. I'm certainly no expert on color, but I'd guess that the difference you're talking about is probably due to the difference between RGB and RYB or CMYK color models. Go to a paint store and they'll tell you that the three primary colors are red, yellow, and blue. On computer screens, obviously, colors are composed of red, green, and blue sub-pixels.