Background colour expression brings up a surprising result

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I have a report where I want a cell colour to change if a duplicate value appears. I have used this expression for Background Colour: IIF(Previous(Fields!CustomerSurname.Value) = Fields!CustomerSurname.Value, "Red", Fields!CustomerSurname.Value) This works fine but if the customer surname is 'Brown' or 'Grey' then the cell colour matches the colour of the surname, eg the surname 'Brown' creates a brown cell background, not red. This doesn't happen with other surnames, eg ones that are not the names of colours... Any tips / guidance on how to avoid this would be appreciated - thanks.