I have no way of testing this, but did you try to format the field in the Inspector>Data Formatting. Select from the Format drop down 'Decimal'. Select 'Fixed number of decimals' 0. Select a color for 'Negative'.
That worked! Ha! I totally forgot I could do that ;) Thanks!
I'm not sure which field I should be using as the "Break Field".
For future reference should you need getSummary in another context, the "Break Field" and your sub summary part's "when sorted by" field should be exactly the same field if you want to access the subtotal value of a summary field placed on that sub summary part. The one limitation to getsummary is that it requires that a calculation field using getsummary, the summary field and the break field all be defined in the same table where the sub summary layout part's break field can be a field from a related table.