0 1 2 3 4 5 6 7 8 9 A B C D E F -- Hexadecimal digits

Hex Binary Decimal
 0   0000     0
 1   0001     1
 2   0010     2
 3   0011     3
 4   0100     4
 5   0101     5
 6   0110     6
 7   0111     7
 8   1000     8
 9   1001     9
 A   1010    10
 B   1011    11
 C   1100    12
 D   1101    13
 E   1110    14
 F   1111    15


RGB - Red  Green  Blue

8 bits Red  8 bits Green 8 bits Blue
 00000000    00000000     00000000
0000 0000   0000 0000    0000 0000
1111 1111   0000 0000    0000 0000
   F    F      0    0       0    0
        
             #FF0000

8 binary digits for Red   2^8 = 256 shades of Red
8 binary digits for Green 2^8 = 256 shades of Green
8 binary digits for Blue  2^8 = 256 shades of Blue
========
24 bits of color  2^24 = 16.8 million unique colors


00 hex = 0 decimal
FF hex = 255 decimal