A typical color definition in HTML would look like this:
The color is defined to be "#FF8C67".
The # simply states that the following numbers are hexadecimal.
The first two digits are the amount of red (FF).
Digits 3 and 4 specify the amount of green (8C).
Digits 5 and 6 specify the amount of blue (67).
Below is an example showing how different colors are mixed to obtain the desired result:
Since there are two hexadecimal figures for each color you can specify 256 gradients of each basic color. This gives a total of 256*256*256 = 16,777,216 different combinations.
However, not all of these colors will look the same on all browsers and operating systems.
Proceed to learn about the 216 colors that are "safe"...