Go to your ELS settings file in your GTA IV folder and scroll down to the cars you want to change the lighting reflection for. If you want Blue change the 0 to 1. For example below this is one of my cars in the ELS settings file. The left half of the light bar will cause a red reflection and the right will be blue.
EX:
[ POLICE ]
Active = 1
Halogen_Only = 0
Color_Light1L = 0
Color_Light2L = 0
Color_Light3R = 1
Color_Light4R = 1
Color_Light7L = 3
Color_Light8C = 3
Color_Light9R = 3