You need to use the following code :
chWorksheet:range("a1"):font:colorindex = INT.
where chWorksheet = COM-HANDLE to the worksheet.
INT = an integer value representing a color in Excel.
a1 = reference to the cell you wish to change the color.
You can always find this type of code by recording a Macro in Excel and doing what you want to program manually. The code, including properties such as above, is then spelled out in the Macro.
Hopes this gets you going in the right direction.