In the standard #RRGGBB notation, there are 256^3 color combinations available, or 16,777,216.
This is because each color value RR, GG, BB can contain 256 different values ranging from 00 to FF.
Therefore, the number of combinations is 256^3.
Many modern browsers also support transparency. This means there is a whole new set of color codes in the form of #AARRGGBB. In this case, there are 4,294,976,296 color codes, or 256^4. It is important to keep in mind that the addition of transparency does not indicate that there are additional colors, because just adding transparency does not change the color. It merely adds more information and more available color codes, depending on the transparency, or level of opacity.