In the HSB (Hue, Saturation, Brightness) model used in most design tools, hue is expressed as a degree on a 360° colour wheel: 0° is red, 120° is green, 240° is blue, and so on around the spectrum. Saturation controls how much of that hue is present; brightness controls how much light it appears to emit. The three work together to describe any displayable colour, but hue is the one that carries identity.
Adjacent hues on the wheel create analogous colour palettes — harmonious and unified. Hues opposite each other are complementary — high contrast, vibrant, and attention-commanding. Hues separated by 120° are triadic — balanced and dynamic. These relationships are not arbitrary; they emerge from the physics of how wavelengths of light interact with each other and with human colour perception.
In semantic colour systems (as used in design tokens and Material Design), hue choices are normative. Blue for information, red for error, green for success, amber for warning — these conventions have become so widespread in digital product design that violating them requires conscious justification. A red success message will confuse users not because red is wrong in the abstract, but because the convention is so deeply embedded.