Audemars Piguet used tritium as its luminescent material up until the year 1999. Energy is released in tritium's beta decay, and lume used on the markers for hour and minute scales of Ref. 5402ST is prone to changing tone due to this factor. It's particularly noticeable if the watch has had its hands changed, where the fresher slightly green tone of the lume on the hands differs from the aged lume on markers.
You can also tell if the hands have been changed on Ref. 5402ST by comparing the thickness of the hands and the markers. The so-called "slim" hands typical of the original composition on Ref. 5402ST are about the same thickness as the markers on the hour scale. This is a subtle yet strong sign of the highly aesthetic design used for Ref. 5402ST.