You separated it wrong. It says that hue is what enables you to classify a color as being red, yellow, green, blue, or where it is between any two of those colors that are adjacent in the list, then says that you should also investigate the definitions of brightness, lightness, and saturation. You mistook the intermediate phrase to mean between any colors, whereas it included the word "these" to indicate that it only meant the previously mentioned colors, and then mistook "BRIGHTNESS 2, LIGHTNESS 2, SATURATION 4" to mean that those were alternate definitions, which they were not.
Black is not between any pair of colors; it is without hue, as its saturation of 0 indicates.