Use the following UDF:
Public Function BColor(r As Range) As Long
BColor = r(1).Interior.ColorIndex
End Function
User Defined Functions (UDFs) are very easy to install and use:
1. ALT-F11 Â brings up the VBE window
2. ALT-I
  ALT-M opens a fresh module
3. paste the stuff in and close the VBE window
If you save the workbook, the UDF will be saved with it.
To remove the UDF:
  1. bring up the VBE window as above
  2. clear the code out
  3. close the VBE window
To use the UDF from Excel:
=BColor(A1)
To learn more about macros in general, see:
http://www.mvps.org/dmcritchie/excel/getstarted.htm
or
http://www.cpearson.com/excel/WritingFunctionsInVBA.aspx
for specifics on UDFs