|It sounds like Conditional Formatting will work for you.|
Select the cell(s) that you want to highlight.
1 - From the Ribbon select Home - Styles - Conditional Formatting,
2 - From the drop down select 'Manage Rules' and select 'New Rule'
3 - In the dialog box select the last item in the list 'Use a formula to determine which cells to format'
4 - Enter your formula in the box.
Something like =IF(TODAY()>A1,TRUE,FALSE)
5 - Note that TRUE and FALSE do not have double quotes - they are Excel's logical values true and false.
6 - Click the format button and from the 'Fill' Tab select a color
7 - OK your way out.
Note: For this to work, Excel has to recognize the formatted cells as dates.