Explanation of hex colors.

gistfile1.txt

HTML colors are broken into 3 components:

R: Red
G: Green
B: Blue

These 3 components may be represented in hexadecimal. 
A hexadecimal representation of the 3 components is called a triplet, or group of 3 numbers.

          R G B    R     G     B
Black:  #000000 0x00, 0x00, 0x00
Red:    #FF0000 0xFF, 0x00, 0x00
Green:  #00FF00 0x00, 0xFF, 0x00
Blue:   #0000FF 0x00, 0x00, 0xFF
White:  #FFFFFF 0xFF, 0xFF, 0xFF

In decimal, the digits are 0-9.
In hexadecimal, the digits are 0-F, which is equivalent to 0-15 in decimal.

In decimal, the rightmost digit are the ones.
Likewise in hexadecimal.

In decimal, the 2nd digit from the right are the tens.
In hexadecimal, however, these are the 16s.

Each component is a two-digit hexadecimal value.
0x00 is equivalent to 0:
  0 * 1   = 0
+ 0 * 16  = 0
-------------
          = 0
          
0xFF is equivalent to 255:
  15 * 16 = 240
+ 15 * 1  =  15
---------------
          = 255

From this simple conversion, we can easily figure out the total number of possible colors. 
The digits are the powers of 16, with the least significant on the right, and most significant digit on the left.

16^5 = 1048576s  16^4 = 65536s  16^3 = 4096s  16^2 = 256s  16^1 = 16s  16^0 = 1s
              F              F             F            F           F          F
   15 * 1048576  = 15728640
   15 * 65536    =   983040
   15 * 4096     =    61440
   15 * 256      =     3840
   15 * 16       =      240
+  15 * 1        =       15
---------------------------
                 = 16777215 color possibilities

You can also watch this free lecture on Coursera by Nick Parlante from Google: https://class.coursera.org/cs101-2012-selfservice/lecture/2

math woot!