Make sure you set the color value in the else part. Here is code I use to set the background color for a row:

=IIf(Parameters!Program.Value = "ALL",

Switch(Fields!CurrentClub.Value = 1, "DarkGray"

,Fields!CurrentClub.Value = 5, "DeepPink"

,Fields!CurrentClub.Value = 10, "Red"

,Fields!CurrentClub.Value = 15, "Yellow"

,Fields!CurrentClub.Value = 20, "LightGreen"

,Fields!CurrentClub.Value = 25, "Turquoise"

,1=1, "White"

), "White")

I set the background to "White" when it should not be highlighted, I tried setting to a NoFill type value but couldn't quite get that to work.

Try the above and see if that resolves your issue.