Ceril

If you're using ELS, then you can find the colors in the els config file located in your GTA IV directory. If you scroll down there should be a place in the document set up like this:
 
[ POLICE ]

Active = 1
Color_Light1L = 0
Color_Light2L = 0
Color_Light3R = 1
Color_Light4R = 1
Offst_SLights = 0
 
the number is the color of the light, which corresponds to this chart (also at the bottom of els.ini):
Red         0
Blue        1
White      2
Amber     3
Green      4
 
The look of your file might vary slightly based on the version of ELS you're using.
 
Hope this helps!