There is an easier way to make any engine any color plus you can change engine color and cruise/contrail color.
Easieast way is to open up Engine-Equipment.ini
find your ship engine and look for the line "flame_effect="
Put the flame effect of any engine in the whole game their to get it on your ship.
To change the cruise color/trail color find the line "trail_effect=" and "trail_effect_player=" these two lines have to be the same and will only work if the effect you insert ends with _trail
Heres an example from my liberty cruiser
[Engine
nickname = cruiser_engine_01
ids_name = 263887
ids_info = 264887
volume = 400.000000
mass = 10
max_force = 300000
linear_drag = 3125
reverse_fraction = 0.100000
flame_effect= gf_ng_largeengine01
trail_effect= gf_br_transport_engine01_trail
trail_effect_player= gf_br_transport_engine01_trail
cruise_charge_time = 5
cruise_power_usage = 20
rumble_sound = rumble_cruiser
rumble_atten_range = -5, 0
rumble_pitch_range = -25, 25
cruise_start_sound = engine_li_cruise_start
cruise_loop_sound = engine_li_cruise_loop
cruise_stop_sound = engine_li_cruise_stop
indestructible = false
outside_cone_attenuation = -3
inside_sound_cone = 90
outside_sound_cone = 270