Why is the hexadecimal color different between xaml page and web page?

Nobody said, that the color values in wpf are html color codes. Look here: http://msdn.microsoft.com/en-us/library/system.windows.media.brushes(v=vs.85).aspx[^]
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Quote:

XML

<!-- Note that the first two characters "FF" of the 8-digit
     value is the alpha which controls the transparency of 
     the color. Therefore, to make a completely transparent
     color (invisible), use "00" for those digits (e.g. #000000FF). -->

So, those color codes are RGBA[^] codes. And now about html: CSS3 is supporting alpha cahannel in, but not in hexadecimal code (see[^]).