How to make data in cells mandatory?

Hi Dom, Yes, the cells are colour coded using conditional formatting.
I have attached a sample workbook test.xls

In A1 of sheet1, Data can be selected from drop down menu.
Based on which, I have done some conditional formating to show which cells are mandatory to fill by colour coding the cells in RED.

All I am looking is, If a person selects a data in A1 from the drop down & a couple of cells are highlighted RED(meaning some of the required information is incomplete), he SHOULD NOT be able to save this file(test.xls) & also I want an ERROR pop-up displaying
"Some of the mandatory fields are blank.
Please fill all the fields in RED to save your work"

Thanks
Raj