RGB – Red, Green and Blue is the Color Model (Mode) that is used for images that are displayed on any kind of screen: monitor, TV, video and digital cameras, scanners etc.
A large percentage of the visible color spectrum can be represented by mixing red, green, and blue colored light in various proportions and intensities.
When all three RGB colors illuminate with their highest intensity, together they generate the color white. If they illuminate with their lowest intensity they produce the color black. Here one refers to additive mixing of colors, as opposed to the subtractive mixing of colors, which is applied in the CMYK model.
Web Safe Colors
Initially web designers only used a limited palette of 256 colors which in fact was reduced
to 216 when selecting colors that were common to both platforms. It is no longer necessary for web designers to be so restricted, very few if any of today's monitors are still have these limitations.
Web Smart Colors
Web Smart Colors consist of a palette that includes 4096 colors. Once monitors started to display thousands of colors web designers had a lot more choice.
Millions of colors
Although today designers often choose to use the millions of colors palette, not all devices read millions of colors correctly.
The hex number for both web safe and web smart colors is represented in pairs. In the CSS the hex number can be represented using only one digit from each pair.
Example: #669900 can be written as: #690.
Green, Blue, Red
Cyan, Yellow, Magenta
Secondary colors are created by combining two primary colors.
The value for each color is between 0 - 255. When you are using an application such as Photoshop the combination of all three rgb colors is then assigned a hex number. Color Chart