Green, orange, yellow, and blue — each of these is a hue, a color or a shade that's true. A rainbow shows the melting of one hue into another, from red to violet, and all shades in between.
The noun hue means both a color and a shade of a color. Green is a hue, and turquoise is a hue of both green and blue. Most of the time the word hue refers to colors, but sometimes it's used for shades of meaning or even the tone of a person's face, as in "The speech had a hue of politics — disguised with humor — that turned the mayor's face from a pale hue to red as he laughed."