Engines are available at most bases. When you scan ships in space you can see their engine and get an idea of the color they produce.
There don't seem to be any large differences in performance, but it's sometimes worth it to buy an engine to get the right "look"-- that red and blue Border Worlds flame, that Rheinland green, those Liberty smoke rings, etc.
That being said, the descriptions and pictures rarely tell you anything about the color.
There are over 300 engine entries, not all of them unique, so I will try to post engine screenshots as I get them.
Hopefully someone finds it useful!
HN-D6 Engine Mk I (Class 5)
Red flame, blue trail
gf_co_smallengine01_fire
gf_co_smallengine01_trail
NOTE: Same flame and trail for HN-D4 Engine Mk I and HN-D6 Engine Mk II
PT-HN D9 Engine Mk 1 (Class 3)
Green flame, green trail
dsy_gai_fire dsy_gai_trail
PT-HN D9 Engine Mk 1 (Class 2)
Green flame, green trail
dsy_gai_fire dsy_gai_trail
NOTE: Same flame and trail for PT-HN D4 Engine Mk I, PT-HN D8 Engine Mk I and PT-HN D9-I Engine Mk I
ZT-170 Engine Mk I (Class 5)
light purple flame, smoke circle trail
gf_li_largeengine01_fire gf_li_largeengine01_trail
NOTE: Same flame and trail for ZT-170 Engine Mk II