I set some cells in a workbook based on their value to color CLWebPaleGreen. When the file is exported to XLS, the cells are colored, but when Excel opens the workbook, they are grey, not green. Cells colored clWebPaleVioletRed comes out looking like a red-coral.
I worked out my answer through trial and error. Basic colors seem to be OK, anything newer than the mid 90s seem to just be grey toned out.
The possible colors to use in an XLS file are limited. We try to map a color in the grid that can be a 24bit color value to the closest color in the range of 64 colors the XLS file format supports.