Every designer has been here. You have one good brand color, maybe a blue you love, and now you need four more that go with it. So you open a color picker, guess a lighter blue, guess a pink that might work, and twenty minutes later your palette looks like five strangers at a bus stop.

That guessing is the problem. Color harmony is not taste, it is math, and the color wheel has done the work for you for about three hundred years.

The wheel is a calculator

A color is really three numbers: hue (where it sits on the wheel, 0 to 360), saturation (how vivid it is) and lightness (how bright). Harmony rules are just recipes for picking hues at fixed distances from your base color.

Complementary: your color plus the one exactly opposite it on the wheel, 180 degrees away. Blue and orange, purple and yellow. High contrast, high energy, and the reason every "50 percent off" banner you have ever seen is orange on blue.…