What is a HEX color — #RGB and #RRGGBB
What a HEX color is: short form, HEX8 with alpha, how it maps to RGB, and an online converter.
A HEX color in CSS starts with # and stores red, green and blue as hexadecimal digits. #ff7e15 is the three bytes 255, 126, 21. The short form #f80 expands each digit to #ff8800.
Eight digits (#rrggbbaa) add alpha. Names such as tomato are also valid CSS. RGB and HSL describe the same screen color on other axes; OKLCH is easier for “10% lighter” in modern CSS.
The ToolsHelper picker accepts HEX, a name and rgb() in one field. Contrast next to it follows WCAG against white and #000000.
A hex colour is a base-16 RGB value. #ffffff is white, #000000 is black, and #ff7e15 is the orange used on this site. Two digits per channel run from 00 to ff, which is 0 to 255. A three-digit form repeats each digit. Eight digits add alpha at the end. This page defines the notation. Converting it to the rgb() function is the converter.
FAQ
Is HEX the same as RGB?
Yes, the same three channels, written 00–ff instead of 0–255.
Why HEX8?
So you can set transparency without a separate opacity property.