Worksheet problem in excel 2003

I have a worksheet that I have a yes,no drop down menu. I then have 4 cells below that, that currently have conditional formatting to them that change them to red when Yes is selected in the above drop down box. The red in the 4 cells tells the person that is filling out the worksheet that because you entered yes on above question you now have to fill in the information that is now needed below. Now as they answer the questions to the 4 cells below as the cells get value put in them I would like them to turn to white after the question is answered. Also I would also like to have code written that if yes is answered and the person that is filling out sheet fails to put a value in each cell the questions error msgs saying you forgot to enter the answer question # 4 or so on and so on. Help me I have been struggling