I finally figured it out and it was such a simple solution. Here's the expression I used.
In the detail cell:
=IIF(ISNOTHING(SUM(Count(Fields!RevCategory.Value))), 0, SUM(Count(Fields!RevCategory.Value)))
In the Fill section of the detail cell:
=IIF(ReportItems!RevCategory.Value = "ERP(Sys)" and ReportItems!RevCategory1.Value >= 0, "LightBlue"
, IIF(ReportItems!RevCategory.Value = "Add-On Count" and ReportItems!RevCategory1.Value >= 0, "YellowGreen"
, "PeachPuff"))