Just finalized a measurement of another LED, a cyan Avago HLMP-CE26-WZ000, material = InGaN.
Results:
Avg. temp. (C) / Avg. LED voltage (V)
20,463 2,76333
25,888 2,74719
31,672 2,73049
36,405 2,71664
41,711 2,70043
Again, the voltage and temperature numbers are averages over a period of 10 minutes once the temperature had settled.
The resulting voltage temperature coefficient for this LED turns out to be -2.95 mV/K.