RGB to hex converter: turn three 0-255 numbers into a hex code

Base 16 uses the digits 0 to 9, then A to F for 10 to 15, so a pair runs from 00 to FF. The first digit of a pair counts 16 times as much as the second. Take the gray #808080. Its green channel is 128, which is 8 × 16 + 0, so the pair is 80.

Every pair needs two digits, even when the first one is zero. Take rgb(5, 10, 15): its channels are 5, 10 and 15. All three sit below 16, so each needs its leading zero. The code is #050A0F. Drop just one zero and the code has five digits, which CSS doesn't accept. Drop all three and you get #5AF, which CSS reads as #55AAFF. That's a valid code for a different color, so no error warns you.

The three-digit form works only when each pair repeats one digit, like FF, 00 or 88, which is why #FFFFFF can be written #FFF. Case doesn't matter either. The RGB to hex step is exact, since each 0 to 255 number maps to one pair and back, but the HSL and CMYK rows round their numbers. Going through HSL returns #21C45D for this color, not #22C55E, so a round trip through HSL can land on a slightly different code.