LED behavior description
From fit-PC wiki
LED-R (RAM) This LED turns Green, the moment the memory initialization is successful. This LED is Yellow while the memory initialization is in progress. If the memory initialization has failed, for example: There is no DIMM installed / the DIMM is not supported - the R LED will stay Yellow. The R-LED should stay Green at normal operation, also after BIOS POST is over.
LED-B (BIOS) This LED is Yellow after display initialization. If BIOS setup menu is needed, the DEL button is functional from the moment the B-LED is Yellow. The B-LED is Green when the OS takes charge of the system.
LED-H (HDMI display) This LED is Green only when the HDMI port of the internal graphics is connected to a monitor.
LED-D (Display-Port display) This LED is Green only when one or both of the Display Ports of the internal graphics are connected to a monitor.