The Rule of 60-30-10
A classic interior design rule that applies perfectly to UI: 60% neutral (backgrounds/surfaces), 30% secondary (borders/headers), and 10% accent (buttons/links). This balance ensures your site doesn't feel overwhelming.
Analogous for Harmony
Analogous colors (those next to each other on the color wheel) create a serene and comfortable design. For a blue primary, consider using a cyan or violet as your secondary color for a cohesive look.
Complementary for Contrast
If you want specific elements to "pop," use a complementary color (opposite on the color wheel). A blue button on an orange-tinted surface creates instant visual hierarchy and draws the user's eye immediately.