Excel and SSRS 2005 colors don’t always match up the way you want them to, so here is a comparative analysis that will hopefully help you select colors that match up well.
Some of the colors that match up well:
Black, White, Gray, DarkGray, Gainsboro/LightGrey, Maroon/DarkRed, Red, Brown/SaddleBrown, Salmon, LightCoral, OrangeRed, LightSalmon, Sienna, Peach Puff, Dark Orange, BurlyWood, NavajoWhite, Orange, Wheat, LemonChiffon, Cornsilk, Gold, LightGoldenrodYellow, Olive, Yellow, LightYellow, YellowGreen, ForestGreen, Green, Lime, Turquoise, MediumTurquoise, MediumSeaGreen, DarkSlateGray, DarkCyan, Cyan/Aqua, SlateGray, Royal Blue, Midnight Blue, Navy/DarkBlue, Blue, DarkSlateBlue, MediumOrchid, Purple, Fuchsia/Magenta, Crimson, Transparent
In 2008 they load a custom color palette based on the colors used in the report, so the colors will always match what is in your report. While this is a nice feature I have found one bug that occurs when merging two spreadsheets that have been rendered through SSRS. The color palette of the main workbook will be applied to all other sheets that are added to it, and they appear to use some sort of index to reference the color. So if you have two SSRS rendered report with different colors and you attempt to merge them into one, the colors on the added sheet will go haywire. You can try to match the colors between the two SSRS reports, but even having the order of the colors in the report off will give you some strange results (presumably because the index of the color in the palette is used.) So watch out for this!