I don't actually know why the colours are not different for you - they should be - the first colour in this palette should only be used for the first expression and so on...
I would, however, propose a different approach:
=> For the sake of simplicity, I follow a (self-made) internal standard here to never use the colours_tab, but always define expression_colours via the expression_properties (which you can expand by clicking on the + on the "expression" tab)
P.S.: Nizam is probably right. Have you maybe just copied one expression over and over and then adapted it? - if so, any background_colour that was set in the expr_properties would have been copied of course - and those settings override anything you do on the "colours"_tab.