Toggle Interior.ColorIndex using mouse clicks

I have found the following code that changes the colour of a cell when clicked to Red


This is fine, but what happens if I click the wrong cell?

What code would do the following:

Click a single cell
Check the cell has [No Fill]
If [No Fill] then fill [Red]
If [Red] then reset to [No Fill]

I think I have the logic of what I want to do, but not the knowledge of the code required.
Which is where you experts come in.

Thanks for your time

Neil