In more detail
Classic schemes use positions on the colour wheel: complementary (180° apart), analogous (neighbours within about 30°), triadic (three hues 120° apart) and monochromatic (one hue at several lightness levels). Working in HSL makes this easy, because only the hue angle changes. Practical palettes also include neutrals for text and backgrounds, and a common guideline splits use roughly 60% dominant, 30% secondary and 10% accent.
Why it matters
A palette keeps a site, brand or chart consistent and saves choosing colours one by one. It must also work in practice: text and background pairs need enough Colour contrast, and categories in a chart should stay distinguishable for people with Colour blindness. A palette that looks good as swatches can fail both tests.
Example
Starting from #3366cc, which is hsl(220, 60%, 50%):
- complementary (hue 40):
#cc9933 - triadic (hues 340 and 100):
#cc3366and#66cc33 - analogous (hues 190 and 250):
#33b2ccand#4c33cc
As text on white, #3366cc reaches 5.37:1 and passes AA.