How to get the correct color marks associated with Month over Month values?

I am having trouble with my color marks being associated with my Month over Month 2 calculated field.

As seen below, the highlighted numbers should be green to signify an increase instead of a decrease (red).

Here is my calculated field: ((ZN(([Metric]))) - (LOOKUP(ZN([Metric]),-1)))

Is there another way to calculate month value - the previous month?