FF0000, as only the red color, has a value for brightness and as it is the highest value possible (f=16), this color code shows the brightest red.
00FF00, now the red color has no value, but the green color has, meaning this is the most brilliant green color.
0000FF, same concept here only the last primary color (blue) has a value for brightness and shows, therefore, the brightest blue color.
In CSS it is possible to use a shorthand three-digit format, where the single digits get doubled. So for example 0FC would be 00FFCC in the usual form. In the Shorthand format, there are only 4096 possible colors.
To use a Hex color in CSS the following format needs to be used
#RRGGBB. Hex colors are supported by all browsers.