Change Color Of Box On Report Based Off Value

What I'm trying to do is make it so that a certain area of my Detail area in my report changes colors based on whether or not a field name for each record is set to "Yes". The effect I wanted was produced easily enough by creating a Rectangle, coloring it Red, and setting it to not visible.

Putting RedBox.Visible = True in the Report On Load procedure works just fine. The problem I am having is attaching a condition to it.

Code:

If Me.FormatBox.Value = "Yes" Then
RedBox.Visible = True
End If

This compiles ok, but doesnt do anything to the actual report. Will it not work because the report has x amount of FormatBox's because it gets replicated for each record that is displayed? I should also note that I tried using a DL Lookup within my If statement and that didnt work either. That just colors all the boxes red and ignores the check.

Code:
Dim TestVar As String
TestVar = DLookup("Priority", "Table_Lancaster_Dispatch", "Priority = 'Yes'")
If TestVar = "Yes" Then
RedBox.Visible = True
End If