That's a lot more work than you think (to do it fully anyway), & can't be done in 'visualsettings.dat' unfortunately.
Have a look at the files in White LED Streetlights With HDSRL Lighting Technology to get an idea of what is involved in changing the streetlight's colour completely.
If you don't mind light fittings & lod lights being orange while having white streetlighting, easiest method would be to change the rgb values for the '<light_artificial_ext_down/<light_artificial_ext_up' values in each of the timecycle weather files ('w_extrasunny.xml', 'w_clear.xml'... etc) & set them equal (ie black/grey/white depending on how high the value is).
Results may vary/look a little weird/orange light fittings producing white light etc.
Easiest method would be to lower the 'postfx_exposure' values in the timecycle weather files
(hint: you'll be editing the 1st value (00:00 > 05:00) & the last (13th) value (22:00 > 00:00) for night-time. Read this post/thread for a bit more info)
<postfx_exposure> = Overall/base postfx exposure value (higher values = brighter, lower values = darker (can be negative if required)).
<postfx_exposure_min> = Minimum (darkest) postfx exposure value
- Raise this value to make the darkest areas of screen brighter
- Lower this value to make the darkest areas of screeen darker
<postfx_exposure_max> = Maximum (brightest) postfx exposure value
- Raise this value to make the brightest areas of screen brighter
- Lower this value to make the brightest areas of screeen darker
Basically, use the <postfx_exposure> value to set the darkness you want & if you have issues with areas of the screen that are too bright or too dark, use the '<postfx_exposure_min>' &/or '<postfx_exposure_max>' values to tune them to your liking.