Prevent save if certain cells not filled (Mandetory cells dependent on dropdown list)

Hello All,

I would be grateful if you could assist me on a problem I’m facing.

I have a dropdown list in cell a2 (1,2,3,4,5).

I would like cell b2 to be mandatory to populate if 1 is selected in cell a2 (cell b2 should be highlighted to red and automatically changed to white when populated).
I would like cell c2 to be mandatory to populate if 2 is selected in cell a2 (cell c2 should be highlighted to red and automatically changed to white when populated).

And so fourth….

To make this more difficult, I want to prevent save if a number is selected from the dropdown list. I would also want this formula to work for the entire Column a/worksheet.

I trust you Excel professionals can help me. I’m using Excel 2007.

Many thanks.