True color?
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24-bit RGB color is sometimes known as 'True Color', although as we discussed in the section on hex triplets it still falls well short of covering all of the color range visible to the human eye. Nevertheless, 16.8 million possible colors per individual pixel is impressive – although not always achievable. Lets look at this color range and then explore some of the reduced (but possibly safer) color sets.
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Color tables ...
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 | SATURATED RED (FF,XX,XX)
| blended to green (vertical axis)
and blue (horizontal axis).
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|  | SATURATED GREEN (XX,FF,XX)
| blended to blue (vertical axis)
and red (horizontal axis).
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|  | SATURATED BLUE (XX,XX,FF)
| blended to red (vertical axis)
and green (horizontal axis).
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|  | ZERO RED (00,XX,XX)
| blended to blue (vertical axis)
and green (horizontal axis).
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|  | ZERO GREEN (XX,00,XX)
| blended to red (vertical axis)
and blue (horizontal axis).
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|  | ZERO BLUE (XX,XX,00)
| blended to green (vertical axis)
and red (horizontal axis).
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These six panels (click to enlarge) each contain 256 colors created using hex triplets and each individual color square is eleven potential combinations away from each other (in other words, each panel could contain 65,536 colors if every combination was used). Each panel is also effectively one of the six-sides of a cube that could contain all 16,777,216 possible 24-bit color combinations. Unfortunately, as a web publisher, you do not always have all 16.8 million colors at your disposal.
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Using 24-bit color - limitations on the web ...
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There are at least three issues that mean in many circumstances, the available color choice (palette) is substantially smaller than the 16.8 million 'true color' 24-bit RGB range (gamut). Lets have a brief look at file format issues, data size and 24-bit color itself.
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File formats.
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When we take a closer look at image formats, we will discuss their individual strengths and limitations. Suffice to say for now that the three key file formats on the web, GIFs, JPEGs, and PNG files are all limited in some way and the effective end result is that the full array of colors are not used. In the tables above, the file format is a GIF (Graphics Interchange Format, developed by CompuServe), that has a maximum of 256 colors (8-bit). The image creates the 'illusion' of the 24-bit colors by blending adjacent pixels together to simulate the appearance of the missing colors. This is called 'dithering', a process that adds 'noise' to the image and reduces its sharpness, as can be seen by the fact that the small squares in the above tables should be consistent in color if they were accurately represented by the image format.
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Data size.
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Less specific than file format, is the simple notion of data volume. To represent every pixel on a 1024 x 768 pixel screen (0.8 mega-pixels) with 24-bits of data is a total of 2.4 megabytes (Mb) every time the screen refreshes itself. Even on a very high speed internet connection, such a data load would be problematic. For this reason, file formats and web browsers use compression and other rendering techniques to reduce the data transfer required. Many of these techniques result in a compromise between speed and quality of the rendered page.
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24-bit color.
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When the six-digit hex triplet is converted into a binary number, it is a 24-digit long string (2 to the power of 24 equals the same 16.8 million possible combinations), hence the term 24-bit. Although many devices can handle 24-bit color, not all of them can, and as we discuss many times on this site, it is often the 'weakest link' that needs to be addressed if we are going to have a level of consistency amongst the people that are consuming our published content. Some devices can handle 16-bit color (also know as 'high color' and including 65,536 possible combinations), however there are still devices and environments that are limited to 8-bit color (only 256 combinations contained in an 8-digit long binary string). Worse yet, since 40 of these colors used to display differently between PC and Apple computers, a consistent set was limited to 216 colors, known as web safe colors.
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