It depends on the type of lighting your vehicle relies on, but here is the method for both ELS and non-ELS:
• For an ELS vehicle it's as simple as opening the corresponding XML file and changing the "Color" value (as demonstrated in the first picture).
• For a non-ELS vehicle access the vehicle's carcols.meta file and adjust the "color value" value (as demonstrated in the second picture).
I make no guarantee that this is right, but I believe these are the hex values used in a carcols.meta file:
Red = <color value="0xFFFF0000"/>
Blue = <color value="0xFF0000FF"/>
Orange = <color value="0xFFFF5000"/>
Yellow = <color value="0xFFFFFF00"/>
Green = <color value="0xFF00FF00"/>
White = <color value="0xFFFFFFFF"/>