Colour models
HEX and RGB describe the same sRGB triplet — this page is an RGB color converter, a HEX color converter, an HSV color converter and a HEX color picker. Changing any field updates the others: RGB to HSL, HSL to RGB, RGB to CMYK and CMYK to RGB. HSL and HSV rearrange the triplet into hue, then either lightness or value — handy for generating palettes by rotating hue. CMYK here is a simple screen approximation (not a print ICC profile).
Harmonies
Complementary colours sit 180° apart on the hue wheel. Analogous neighbours (±30°) feel quieter. Triadic (120°) and tetradic (90°) schemes are louder and need a dominant colour. Split complementary uses the two hues adjacent to the opposite. Monochromatic keeps hue fixed and walks lightness. Together that is a color scheme generator and color harmony generator — complementary, analogous, triadic, split complementary, tetradic and monochromatic color generators from one base.
Before shipping a UI palette, check body text against WCAG contrast. Dark text on a light card is the usual pass; pale grey on teal often fails AA.