On p1999, I've gained xp from Pumas, willowisps, Grizzly bears, and silvermist wolves, all of which were green (levels 16/17). 
 
If memory and perception serves, light blue was introduced partially as a way to distinguish high green from etc greens (as it affects adds, xp, etc), but some high greens on p1999, even with the light blue con system, give xp - so the difference was in labels, not in the actual exp you gained from the mob. 
 
Quad kiting high greens/light blues was always decent exp because of such low resist %'s and the low mana overhead. 
		
		
		
		
		
		
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