Thanks and indeed "interesting" ... however to come to the conclusion the WS-10 has a "greater thrust compared to AL-31F" based on a lower exhaust temperature and that based on colours is a bit far-fetched!
Even more since today most engines are aimed for even higher temperatures in order to increase their efficiency ... as such a contra-argument could now be to say "the WS-10 has a lower exhaust temperature, and as such a lower core temperature, which would be a hint it is not as efficient and technology-wise "older" or at least less modern!"
I think the higher and hence the better temperature refers to temperature of heated or burned air flow
before the turbine, which is obviously the higher the better, not the jet flows at exhausting nozzles. Turbo fan engines normally have much greater thrust than turbo jet engines with cores on similar specs but with much lower temperature at final jet flows.
The guy who wrote the article iterates through what he saw in the jet flames, which supports his assertion that WS-10 has greater thrust in same or similar working conditions, that WS-10 has
- lower jet flow temperature
- lower speed of the jet flows when the aircraft takes off in relatively low speed (by noticing the Mach loops in the last picture of a J-11A with AL-31F in which afterburners are engaged and evidently supersonic jet flows are visible, ie. more energy is wasted)
Look, if you have a turbo fan engine with higher before-the-turbine temperature and lower temperature after, the heated air will be more pressurised, energy will be converted more from heat into air pressure and hence the thrust will be greater.
Hope that helps.