Conditional format should do what you are after

see link for example it in action
http://office.microsoft.com/en-au/ex...102809699.aspx

also a small video on the steps to take
http://www.youtube.com/watch?v=rGHaRrwT9sw

the difference to this video would be instead of typing the word into the conditional format you would direct it to a cell which is your drop down menu cell