When you are customizing the theme of your community you will notice there is no color picker. Here is a list of the Hex codes you will want to reference when updating the colors of your theme. The Hex code is the first six characters from the table below.
A number of colors are defined by web browsers. A particular browser may not recognize all of these colors, but as of 2005 all modern general-use browsers support the full list of colors. Many of these colors are from the list of X11 color names distributed with the X Window System. These colors were standardized by SVG 1.0, and are accepted by SVG Full user agents. They are not part of SVG Tiny.
The list of colors actually shipped with the X11 product varies between implementations, and clashes with certain of the HTML names such as green. Furthermore, X11 colors are defined as simple RGB (hence, no particular color space), rather than sRGB. This means that the list of colors found in X11 (e.g. in /usr/lib/X11/rgb.txt) should not directly be used to choose colors for the web.
The list of web "X11 colors" from the CSS3 specification, along with their hexadecimal and decimal equivalents, is shown below, compare the alphabetical lists in the W3C standards. Note that this includes the common synonyms: aqua (HTML4/CSS 1.0 standard name) and cyan (common sRGB name), magenta (common sRGB name) and fuchsia (HTML4/CSS 1.0 standard name), gray (HTML4/CSS 1.0 standard name) and grey. 
At one time many computer displays were only capable of displaying 256 colors. These may be dictated by the hardware or changeable by a "color table". When a color is found (e.g., in an image) that is not one available, a different one has to be used. This can be either using the closest color (fast) or dithering (slow, looks better).
There were various attempts to make a "standard" color palette. A set of colors was needed that could be shown without dithering on 256-color displays; the number 216 was chosen partly because computer operating systems customarily reserved sixteen to twenty colors for their own use; it was also selected because it allows exactly six equally-spaced shades of red, green, and blue (6 × 6 × 6 = 216), each from 00 to FF (including both limits).
The list of colors is often presented as if it has special properties that render them immune to dithering. In fact, on 256-color displays applications can set a palette of any selection of colors that they choose, dithering the rest. These colors were chosen specifically because they matched the palettes selected by the then leading browser applications. Fortunately, there were not radically different palettes in use in different popular browsers.
"Web-safe" colors had a flaw in that, on systems such as X11 where the palette is shared between applications, smaller color cubes (5×5×5 or 4×4×4) were often allocated by browsers—thus, the "web safe" colors would actually dither on such systems. Better results were obtained by providing an image with a larger range of colors and allowing the browser to quantize the color space if needed, rather than suffer the quality loss of a double quantization.
As of 2011, personal computers typically have 24-bit (TrueColor) and the use of "web-safe" colors has fallen into practical disuse. Even mobile devices have at least 16-bit color, driven by the inclusion of cameras on cellphones.
The "web-safe" colors do not all have standard names, but each can be specified by an RGB triplet: each component (red, green, and blue) takes one of the six values from the following table (out of the 256 possible values available for each component in full 24-bit color).
|C or (12)||CC||204||0.8|
|F or (15)||FF||255||1|
The following table shows all of the "web-safe" colors, underlining the really-safe colors. (One shortcoming of the web-safe palette is its poor selection of light background colors.) The intensities at the low end of the range, especially the two darkest, are often hard to distinguish.
In the table below, each color code listed is a short-hand for the RGB value; for example, code 609 is equivalent to RGB code 102-0-153 or HEX code #660099.