Recalling what little physics I know, this could be explained by two factors:
The blue color is given by the characteristic EM radiation of C-H bond breaking, C- and H- radicals forming and recombining with O- radicals to form CO2 and H2O. This is independent of temperature and is the reason why a new gas oven's flames are blue.
The orange color is definitely the characteristic blackbody radiation wavelength of about 1500K, ~1200 degree Celsius. Now the problem is what the source of this blackbody radiation is. The way I see it it's possibly from three sources:
The hot afterburner inner wall,
The soot produced by partial combustion, or
The incandescent exhaust itself.
If the above recap is correct, this means either
- the Western afterburners either run a lot dirtier than Eastern afterburners, which is unlikely because F35 and F22 have orange afterburner flames and they must be running squeaky clean to not have their afterburner inner walls coated with soot (amorphous carbon) thus having a huge IR signature because of the high emissivity of carbon, and we do not see black smoke coming from any jet since F4,
- Western afterburners are a lot hotter than Eastern afterburners, to the point that the blackbody radiation color, which is dependent on temperature, dominates the radical color, which is independent of temperature. This seems to be the logical conjecture, given that the Russians have always been pretty poor on metallurgy.
Does anyone have authoritative knowledge on this subject? This rumor seems to be circulating on the internet forever.
OK, the exhaust may seem a little purplish, that is caused by the emissive characteristic of H2O.
Or, maybe the Su-27 and the MiG-29 just had a cold start and the inner walls of their afterburners are still cold?
On closer inspection, the orange color seems to be solely coming from the glowing afterburner's inner wall, as opposed to the exhaust, although the exhaust is glowing also. This seems to confirm my guess that Western afterburners run hotter.
OK, I've spoken to some guys from one of the ex-Soviet design bureaus. One of them said the blue color is the result of cold afterburner and Sukhoi afterburner design, the fuel being dumped at acute angles into the center of the cylinder using a few big injectors.