Colour utilities: ?op=convert (default) turns any hex/rgb()/hsl()/CSS-name into hex, rgb, hsl, hsv, alpha, relative luminance and exact CSS name; ?op=contrast with ?fg=&bg= returns the WCAG 2.x contrast ratio and AA/AAA pass/fail for normal & large text plus UI components (accessibility / EAA); ?op=palette derives complementary, analogous, triadic, tints and shades. Send hex without '#'.