The Details
Digital images are made up of tiny little squares, called pixels. An image contains a static number of pixels, for example, a 1024x768 image would have 1024 columns and 768 rows of pixels. Each pixel can only hold one color; these colors go by a few different scales, one of the most popular is RGB or RRGGBB. These values are expressed in hexadecimal form. The chart below shows how they work.
Experiment
There are two ways to express RGB format, the first is a number for each letter, ranging from 0-255; in this case, 256 would represent transparency.
The second way to express RGB is quite simple, but one must know their hexadecimal system in order to truly understand it. This works by entering a 6 digit hexadecimal number (0-f), these numbers relate directly to the above format, for example: #ffffff would be 255, 255, 255 above, or simply, white. Shorthand may also be used (RGB instead of RRGGBB): #fff.
Though this color system is extremely logical and easy to learn, it does have its flaws. This being that each pixel can only hold one color, this means, when resizing an image, you loose detail. This phenomena is further explained.