Because computers store and handle binary digits (bits) and four bits make one hexadecimal digits or a Nibble! 8 bits is therefore one Byte and 2 Nibbles is also one Byte. Bytes are used to measure memory and storage and are the magic numbers that enable Hex to work its magic!
The Hex magic is that that any four bit number can be represented by one hexadecimal number (as shown in the table below) as a four but number can only go up to a maximum of 15
One bit, can be broken down into 2 X 4 bit numbers (2 nibbles) and each single one of these can also be represented by one hexadecimal number; the 2 parts can be represented by one hexadecimal number for each of the four bit sequences which means 2 X hexadecimal numbers can represent up to 255 (which is the largest number possible using 8 bits!)
0 to 255 (in decimal) , or
00 to FF (in hexadecimal)
Because each of the three colors can have values from 0 to 255 (256 possible values), there are:
256 × 256 × 256 = 2563
= 16,777,216 possible color combinations
(and this is why you see claims of "16 Million Colors" on computer equipment)
The format ("notation") used on web pages is #RRGGBB, where RR is how much Red (using two hexadecimal digits), GG is how much Green, and BB how much Blue.
Roses are #FF0000
Violets are #0000FF
For more inspiration, the two favourite colours I have found using Hex are these two: