Aha. Looks like you could do a level con check in EVENT_KILLED_MERIT (which fires for each player who got credit for the kill), to see if the npc was grey to them or not.
It looks like what you'd check is $npc->GetLevelCon($client->GetLevel())
Here's a dump of the possible con values from common.h:
Code:
#define CON_GREEN 2
#define CON_LIGHTBLUE 18
#define CON_BLUE 4
#define CON_WHITE 20
#define CON_YELLOW 15
#define CON_RED 13
I don't see an entry for grey. Might take some experimenting!