Variable intensity of interior lighting

According to the wiki entry for lights (3.5.19), you can just set the variable to any value >=0, in which 0 is off, .5 is half brightness and 1 is full brightness.

As for the limitation of 4 light source coupling per mesh, there are two tactics that can, in isolation or combined, to an extent, serve as a workaround (assuming that offloading some of the desired effects to [matl_lightmap] / [~nightmap] / [~allcolor] presents no viable alternative).

For one, you may re-declare a [mesh] as many times as necessary (with mutually-exclusive [visible]-setting) for all permissible combinations of active light sources to be enumerated. IIRC, this technique is employed for the first row of seats in one of the vanilla M+R SDs (potentially depending on frame rate, while cycling through the state of interior lighting, one might catch a glimpse of said object disappearing / reappearing).

For two, you may introduce "synthetic" [interiorlight](s), grouping together multiple physical light sources with the intent of bringing their number of combinations down.

An example based on the original interior lighting scheme of the Citaro: Suppose that you'd like the steering wheel to be illuminated by the driver's light alongside 4 of the fluorescent tubes arranged longitudinally, so by 5 light sources total. Suppose additionally that all 5 sources shall still remain individually controllable, so as to be able to e.g. simulate defective units. Here's how you could go about it:

1. As usual, have one [interiorlight] for each of the 5 sources as-is.
2. Introduce one further [interiorlight] for the first two (first on the left and first on the right) tubes represented jointly (so positioned between the two and with a radius sufficient to enclose the originals).
3. Duplicate the steering wheel [mesh] declaration; add a [visible] constraint to each.
4. Script the mesh visibility transitioning, ideally striving for balance between performance (avoiding unnecessary reassignment of the visibility variable) and aesthetics / immersion.

Skeletal implementation:

# standard variant
[mesh]
steering_wheel.o3d

...

[illumination_interior]
1
2
4
5

[visible]
lights_steering_wheel_selector
0

# variant using synthetic interiorlight
[mesh]
steering_wheel.o3d

...

[illumination_interior]
0
3
4
5

[visible]
lights_steering_wheel_selector
1

# 0
[interiorlight]
lights_int_driver
...

# 1
[interiorlight]
lights_int_1_l
...

# 2
[interiorlight]
lights_int_1_r
...

# 3 - synthetic pair of (1) and (2)
[interiorlight]
lights_int_1_lr
...

# 4
[interiorlight]
lights_int_2_l
...

# 5
[interiorlight]
lights_int_2_r
...

' synthetic light fading variable
(L.L.lights_int_1_l) (L.L.lights_int_1_r) && (S.L.lights_int_1_lr)

' lighting -> object visibility:
' - when drivers light or both front-most tubes active, use the mesh
'   treating the latter two as one
' - otherwise use the regular mesh that differentiates between the
'   two front-most tubes
(L.L.lights_int_driver) (L.L.lights_int_1_lr) || s0
' avoiding unconditional reassignment just in case it affects performance
' or leads to flickering
    (L.L.lights_steering_wheel_selector) = !
{if}
    l0 (S.L.lights_steering_wheel_selector)
{endif}

[matl_allcolor] is for modifying material color attributes; it can be attached to a [matl] or [matl_change] / [matl_item] tag.

Quoting its syntax from "OMSI SDK DEU_02_2011 0514.pdf" (to be found under "OMSI-SDK_2DEU_0.02.zip", retrievable here) :

[matl_allcolor]
Diffuse (red)
Diffuse (green)
Diffuse (blue)
Diffuse (alpha)
Ambient (red)
Ambient (green)
Ambient (blue)
Specular (red)
Specular (green)
Specular (blue)
Emissive (red)
Emissive (green)
Emissive (blue)
Power

Its prevalent application is brightening up key meshes in (user-unfocused) AI vehicles' interiors at times of their interior lighting being active, in lieu of the dynamic [interiorlight]s which OMSI refuses to apply in this context (curiously it still applies them to any passengers within such vehicles... go figure).