A Case structure on the block diagram has an output tunnel, but in at least one of the cases, there is no output value wired to the tunnel. If you run this case, LabVIEW does not know what value to place in the output. LabVIEW indicates this error by leaving the center of the tunnel white. The unwired case might not be the case that is currently visible on the block diagram.
Move to the case(s) that contain(s) the unwired output tunnel and wire an output to the tunnel. You also can right-click the output tunnel and select Use Default If Unwired from the shortcut menu to use the default value for the tunnel data type for all unwired tunnels. When the output is properly wired in all cases, the output tunnel is a solid color.