How can we compare string values for a dimension for current month to previous month?

For example -

If I have six sites A, B, C, D, E, F in a Dimension "Site" in the previous month. In the current  month, I have values A, B, D, F, G, H. How can I create a Calculated Field to give the difference of the two months in "Site" as C, E, G, H.

Also I need to show that Sites C and E are removed & G, H are added in the current month in different colours. Is there a way to do it in Tableau?