Cell Font Color
Excel.Range formatRange;
formatRange = xlWorkSheet.get_Range("b1", "b1");
formatRange.Font.Color = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.Red);
Cell Background color
Excel.Range formatRange;
formatRange = xlWorkSheet.get_Range("b1", "b1");
formatRange.Interior.Color = System.Drawing.
ColorTranslator.ToOle(System.Drawing.Color.Red);
More info...
Excel Cell Formatting
Niva