The caboose marker lamps were applied to all cabooses whether road or transfer. They were amber and were flashing. All along the system, the operators in towers that were open were to check to see that the markers were flashing and notify the train dispatcher when they "OS"ed the train. The lights were to be flashing all of the time.
If the lights were not flashing the crew was to be advised also and they were required to place a red flag on the coupler during the day. If a train was departing a yard just before sundown the cabooses were also given a flashing red light to place on the coupler on the rear of the caboose should the amber light fail.
They were amber because they were more visible in fog than the red ones were.
Source: Pat Livingston, Long Time Conrail Employee