Green is a station that is online, available for scheduling and not in test mode.
Yellow is a station that is online, available for scheduling and in test mode.
Test mode is set when the station owner decides that the station doesn’t perform as it should and needs testing, this could be testing of different configurations, for example different gains, testing of different setups, for example testing different SDRs or Antennas.
An observation that is performed from a station in test mode, is marked as experimental, giving the opportunity to people that are interested in this observation to know that this may not have come from a station that performs the best. For example someone that wants to study/research the performance of a satellite, for better results may avoid using experimental observations.