Introduction
The Interior Reference of an Active Cell can be used to access the ColorIndex property of a cell. The Report Generation Toolkit for Microsoft Office can be used in conjunction with Invoke and Property Nodes to quickly acquire Excel ActiveX references. More information on selectable colors can be found here: http://msdn.microsoft.com/en-us/library/aa199411(office.10).aspx
Refer to http://msdn.microsoft.com/en-us/library/bb258731(v=office.12).aspx for properties and methods of the Interior Reference.