When you have 2 columns and would like to compare the values in each, it is easiest to use a furmula.

Simply use the 'IF' function and provide the 2 cells in each column, then copy the formula down the page.  As an example, to check if A2 and D2 are the same, add this formula to cell E2.

=IF((A2=D2),"same","DIFFERENT")

If the values in A2 and D2 are the same, you will see "same" in E2, otherwise you will see "DIFFERENT" in E2.  Using capitals and a different word will make it easier to see.

As an advanced tip, to change colours of the 'DIFFERENT' cell, use the Conditional Formatting!!