Formula to highlight cell based on data value over 2 cells

Hi Guys and Girls,

I'm am trying to improve a current spreadsheet I'm using where I am collating movement times in and out of a location.

I want to highlight a single cell (cell A) when it has data input, then when data is entered in to a second cell (cell B) I want cell A to return to being in its normal unhighlighted state.

Any help you can provide would be appreciated.