See how different two colors are, whether they read on each other, how they look to colorblind users, and whether they go together — for any hex or color name. Just need text-on-background contrast? Use the contrast checker.
ΔE measures perceived difference. Under ~10 two colors look "basically the same."
WCAG and APCA: text needs about 4.5:1 on its background — and both directions matter.
Check colorblind vision — pairs that look distinct can collapse under deuteranopia.
Complementary, analogous or clashing — the hue relationship sets the mood.