Change the color of cells in one column when they don’t match cells in another column

Select your range from cell A (or the whole columns by first selecting column A). Make sure that the ‘lighter coloured’ cell is A1 then go to conditional formatting, new rule:

Put the following formula and the choice of your formatting (notice that the ‘lighter coloured’ cell comes into play here, because it is being used in the formula):

=$A1<>$B1

Then press OK and that should do it.