Open the vehicles.ide file in your "Grand Theft Auto IV\common\data" folder, find the line beginning with "police" and at the end of that line, add "+livery" (without the quotations).


for IV C:\Program Files\Rockstar Games\Grand Theft Auto IV\common\data



for EFLC C:\Program Files\Rockstar Games\EFLC\common\data

now for more colors.. this line below adds blue and silver to the car if u just want black do not edit and just play but if u want more colors add this line below to the CARCOLS.dat in the "Grand Theft Auto IV\common\data" and just relpace the numbers next to FBI or whatever car u are replacing 

fbi,		10,10,10, 38,55,57, 165,169,167,