I wanted to post this before but just didnt have time:
As Oleg explained, the flame output of an exhaust is affected by factors like temperature and pressure. Flames visibility is due mainly to lighting factors (it'll be harder to see flames on a sunny day), but they're always there (bear in mind that what you see is the output of the avgas combustion straight from the combustion chamber!).
Exhaust fire shouldn't be confused with backfire though, which is an external combustion of a too rich mixture which causes flames like this
Oleg, I love the work you're doing, the attention to details as usual is surprising and comes from a man who obviously has experience with aviation, my humble suggestion is to keep the flames towards the red/blue spectrum more than yellow, since the temperatures involved are far higher than a "yellow flame" when it comes to exhaust output. Another important aspect is to keep them subtle (in terms of transparency/alpha channel) but visible from distance (thinking of the Zerstoerer night fighters spotting the engine exhausts of Lancasters at a certain distance).