Use Conditional Formatting. In Excel 2007/2010:

1) Select the cells in Col A you to apply this to

2) On the Home tab of the Ribbon, click Conditional FOrmatting | Manage RUles

3) Add a new rule, with rule type 'formula to determine format

4) Use a formula of

=COUNTIF(B:B,A2)>0 <assuming A2 is the first cell to be formatted>

and click Format to choose your formatting options