Hexadecimal

Also referred to a “hex” numbers, they are a base-16 numbering system used to define colours online. Hex numbers include the numerals 0-9 and letters A-F. Hexadecimal numbers are written in three sets of hex pairs. Because screen colors are RGB (Red, Green, Blue), the first pair defines the red hue, the second pair defines the green hue, and the third pair defines the blue.

The Html Color Picker is a good place to choose colours.