Why Western fighters' afterburner glow is reddish orange while Eastern Bloc fighters' is blue?
It all depends. I don’t think there’s a distinctive color between the two.   I would submit that the photos of the Flanker may be in a early stage of the afterburners lighting versus the photos of the Raptor or the F-35, which appear to be in full afterburner.   The color of the exhaust gas is a function of the stoichiometric mixture of fuel and air ratio. It full burner the mixture is quite rich, resulting in a orange colored flame.   Lower stages of afterburner are probably  much closer to peak stoichiometric mixture.  Incidentally these blue colored flames are indicative of a much hotter exhaust gas temperature than an orange colored exhaust.   I’m not sure of the reason why one is selected over the other.   I can submit that there are plenty of photos of Russian aircraft in afterburner with orange colored exhaust and other pics of western military airplane with blue colored exhaust as seen below.