They are color codes in hexadecimal notaton.They represent a certain color in HTML documents. Hexadecimal codes are combinaton of three values: red, green and blue. Hence, they have three pairs of two-digit numbers.
The lowest value that can be given to one of the light sources is 0 (in HEX: 00). The highest value is 255 (in HEX: FB).
HEX values are specifed as 3 pairs of two-digit numbers, startng with a # sign.