How to change the colour of a 3rd cell depending on result of 2 others

Good Evening,

Can anybody please help with this. I am trying to change the colour of a cell depending on the result in 2 others.

For example;

If cell A1 has any number in it (1,2,3,4 etc) and cell B1 is empty I need C1 to fill in a colour (initially the colour is not important).

I have tried to write a formula in the conditional format section but have been unsuccessful. It also will need to work in Excel 2003.


Any help or idea are much appreciated.