Mono TIF colour overlay workaround

I’m using data merge for a set of labels. The labels have different colours. I use a path ‘@image from Excel for solid block colours (JPGs) to achieve the different colours in the data merge. I’ve been trying to apply the colours to a white shape (mono TIF) in Indesign with the overlay effect but can’t get it to just colour the white shape  and then not effect the photo in the background. I’ve tried isolation and knockout options but can’t achieve the effect I want. I originally tried setting an object style but couldn't get it to work that way. Is what I’m trying even possible?