This color chart uses only Browser safe colors. These are colors which do not need to be dithered in certain instances.
It is true that we can call as many as 16,777,216 different colors using the RGB HEXADECIMAL values, but most of us could not distinguish the difference between those of similiar enough value. Most of the monitors we use cannot between them. Some browsers we use cannot display all of them, and some net browsers are oblivious to their computer's settings. When non-browser safe colors are called which a browser cannot display, they get dithered. What they do instead of making a space of solid color, is to make a space where some pixels are one color and some pixels another. The effect is very close to the desired color, but the texture, to different degrees depending on circumstances, is different. There are 216 colors which will not be dithered by most Browsers. These are those colors.