Considering you only have one criteria for red, otherwise Green:
=iif(Fields!ColumnA.Value <> "tempdb" AND Fields!ColumnB.Value = "y" AND IsNothing(Fields!ColumnC.Value), "Red", "Green")
You'll need to put this expression in each cell's backcolour property in the row.
Thom~
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk