HEX (e.g., #FF5733) is a compact hexadecimal notation for RGB values used in web design. RGB (e.g., rgb(255, 87, 51)) specifies red, green, and blue components each from 0 to 255, matching how screens emit light. HSL (e.g., hsl(11, 100%, 60%)) stands for Hue, Saturation, and Lightness — it is more intuitive for designers as it separates color identity (hue) from intensity (saturation) and brightness (lightness).

CSS accepts colors in any of these formats directly. You can write color: #FF5733, color: rgb(255, 87, 51), color: hsl(11, 100%, 60%), or color: oklch(0.63 0.21 29). Modern CSS also supports the oklch and display-p3 color spaces for wider gamut displays. HEX and RGB are the most widely supported in older browsers.

CMYK stands for Cyan, Magenta, Yellow, and Key (Black) — the four ink colors used in color printing. Unlike RGB (which adds light), CMYK subtracts light by adding ink. CMYK values are used by graphic designers when preparing artwork for physical printing, where the press uses these four ink channels. Digital screens always use RGB.

Screens use additive color (RGB light), while printing uses subtractive color (CMYK ink). The RGB color gamut (range of possible colors) is wider than most CMYK gamuts — some vivid screen colors simply cannot be reproduced accurately with ink. This is why designs often look slightly duller in print, and why print designers work in CMYK from the start.

Alpha is an additional channel (0 to 1) that controls transparency. In CSS, rgba(255, 87, 51, 0.5) is the same color as rgb(255, 87, 51) but 50% transparent. HSL has an equivalent hsla() notation. The hex format also supports an 8-digit version like #FF573380 where the last two hex digits represent opacity (80 hex = 128 decimal = ~50%).