“Darker color variations are made by lowering brightness and increasing saturation. Brighter color variations are made by increasing brightness and lowering saturation.

[…]

The important bit is this: if you don’t count saturation and brightness, shifting hue towards red (0°), green (120°), or blue (240°) will decrease the luminosity, or perceived lightness of the color. And shifting the hue towards yellow (60°), cyan (180°), or magenta (300°) will increase the perceived lightness of the color.

The trick is to just make the movement of the hue match up with the movement of the saturation and brightness. If you want a darker variation, move the hue towards red (0°), green (120°), or blue (240°), whichever is closest — and vice versa (with cyan, magenta, and yellow) for lighter variations.”