Hex Codes

Hexadecimal (or hex) values, are written as a single string, prepended with a number sign, with digits for each color. #RRGGBB

CSS 
/* Black as a hex code: */
p {
  color: #000000;
}

The major difference is that you can represent 256 values with a two-bit hexadecimal number. In hexadecimal numbering, each digit has 16 values (0-f). And, 16 * 16 = 256.

If we were translate this to binary:

Binary Hex
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 a
11 b
12 c
13 d
14 e
15 f


So, in hex, we write the value:
000  as  00
010  as  0a
015  as  0f
016  as  10
017  as  11
031  as  1f
032  as  20
050  as  32
100  as  64
150  as  96
200  as  c8
250  as  fa
255  as  ff