Changing Cell Text Color When Out Of List Scope

I have a worksheet with 3 particular columns (L,M,N) that have drop down menus. The menu is populated with items that are referenced on a different workbook (different file). Because there are multiple worksheets accessing the referenced list, this is a master. When I change the master, the lists update, but the cell text does not. Example:

If I chose apple from the list, but later in the master I decide I want apple to be banana, while the drop dowm box will now have banana instead of apple, my cell still says apple.

I don't necessarily want apple to change to banana, but I want a visual cue that this is incorrect. I wanted the cell text to change to red if it is not equal to any of the list items on the master file. I want this to be passive, i.e., I want it to change automatically, so I don't have to press a button or some other activation.