It is an attribute syntax with hexadecimal notation to describe the red, green, and blue values of a color, and optionally its transparency.
There are several formats you can use
6-digit notation. The format is #rrggbb, where rr is a two-digit hexadecimal value that describes the amount of red in the color, gg describes the amount of green, and bb describes the amount of blue. For example, #0033FF.
8-digit notation. The format is the same as 6-digit notation, except with two extra values for describing the alpha value, or opacity, of the color: #aarrggbb. For example, #990033FF.
There are also 3- and 4-digit notations for 8-bit color, but these are not commonly used.
r, g, and b are hexadecimal numbers.
For more details you may follow this URL… http://msdn.microsoft.com/en-us/library/system.windows.media.color.aspx
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