Hex colors split a color into red, green, and blue channels, each from 0 to 255, written in hexadecimal. `#FF0000` is full red, `#00FF00` is full green, `#0000FF` is full blue.
Three-digit shorthand (`#F00`) doubles each digit, so it can only represent 4,096 colors, fine for primary colors but not subtle shades.
Eight-digit hex (`#FF0000CC`) adds an alpha channel for transparency, supported in CSS, SVG, and most modern design tools.