|What your looking for is Conditional Formatting,|
see if this works for you:
This is for Excel 2007
1) Select your cell or Range of Cells, B1 - B99
2) On the ribbon click Conditional Formatting
3) Click on New Rules, it’s near the bottom of the dialog box.
4) Click Use Formula to determine which cells to format.
5) Enter the formula: =$B1<$A1
6) Click on the Format button
7) Select the Fill Tab
8) Select a pretty color
9) Click OK
10) Click OK