Best way to do this is to create a calculated field based on the value in question. Right click on the value field in the data window on the left and select 'Create Calculated Field'. In the formula window type
IF [Value] > 0.5 THEN 'Green' ELSE 'Red' END
Replacing [Value] with whatever it is your actual measure is called
Call this new calculated field 'Color based on %' or something like that and then add this new field to the color shelf.
Hope that helps,