-ambient_factor (0-128) This controls ambient lighting which is applied to all parts of the ship from "all" directions. The way this works is a bit weird. (ambient factor * 2) - 255 is "added" to mission ambient, but since this will almost always be a negative value it's actually removed from mission ambient.
Because of how light values are clamped it cannot go below 5.1, ambient_factor 70 will result in -115 to mission ambient which is 120 by default.
70 is essentially the lowest ambient for missions with default ambient settings which are all retail missions and the vast majority of mod missions. Some BP missions have increased ambient so values below 70 actually do something.
So 70 is basically no ambient, 80 is a bit of ambient, and 90 is quite a bit of ambient(ships will look bright from all sides). Anything above that will make ships look incredibly "flat".
Spec_tube and Spec_point are multipliers applied to beam and laser lighting respectively. If you want beams or lasers to light up more set these to around 2. If you want less lighting set them to around 0.5.
Spec_static is a multiplier applied to direct specular lighting from suns. For non-PBR models it will make their sun-lit side very shiny and reflective, for PBR models it will make the sun-lit side a lot brighter. There's no flag to control diffuse static lighting so non-PBR models will always have the same amount of light come from suns.
So for a more dynamic "dark" look you should go with ambient_factor 65, spec_static 2.5, spec_tube 2 and spec_point 1.5. Also disable emissive lighting in the launcher settings because it makes the ships look pretty flat.
This will make lighting "dark" by default but ships will be pretty bright when facing the sun or when firing beams and lasers, resulting in a rather pretty contrasting look.
If you want a flatter, grayer look where lighting is more even use ambient_factor 85, spec_static 0.8, spec_tube 1.5 and spec_point 0.5. Beams should still be pretty bright honestly. Also disable emissive lighting in the launcher.