How to Highlight A Cell when Another Cell Is Selected

I can highlight a cell "A1" if another cell "C1" = 5.

However, I can't figure out the following.
Regardless the value of "C1" I am now trying to highlight "A1" when "C１" is selected by mouse and "A1" won't be highlighted if "C１" is not selected by mouse.
How to make this work?
Your help is much appreciated.

This is not possible with a formula. For this reason rather a question for the VBA forum.

Thread is now in the VBA forum.