Normally measurements are represented in the Browser in black font. However if a measurement is invalid it displays in red font. Invalid measurements can result from various issues including:
The simulation results were deleted, possibly by selecting the Simulation ribbon in the Configuration group.button on the
The chosen dataset contains no data item that matches the configuration of the measurement. For example, a frequency was chosen for a measurement for which no field results were generated in the simulation. Thus the field data is absent from the dataset and cannot be measured.
The best resolution to an invalid measurement is often to change the data source parameters or to reconfigure and rerun the simulation to produce the data you are interested in measuring.