They use HTML Color code's (without the #)
Color Chart.
Look at the chart, pick a color and write down the code, remove the # from the start and thats the code you type into the xml file...
Also alot of graphic programs give you the codes in the palette window, the best program I've seen for getting the codes and color you want is paint.net (which is free), you open the color window and select more, and you can mess around with the color chart and its got lots of slider's and stuff and it displays the exact color code you would need to enter into the xml.
Basic Color Chooser Another Color ChartAnd AnotherSome More Color CodesA Few MoreAnd MoreHope those links help, but as i say download Paint.NET as its very good for getting HTML Color Code (or to be more precise Hexdecimal Color Codes)