I had this issue myself and figured out the solution not to long ago. So I figured I would help those who hadn't figured it out yet. Now just to be clear this is just to help those who have downloaded a police car mod and when loaded into the game the car's police lights only flashed white or a very light red and blue color instead of the full colored red blue the lights were meant to display. In order to fix this issue you will go to the main menu by pressing Esc (escape key) on your key board once in the menu you will want to press the right arrow key until you reach settings then press enter. After that then press the down arrow key until you reach the Graphics tab and then press enter. Then you will press the down arrow key until you see the option to change the setting for post fx as well as  shader. For both of those setting you will set them to high and then that should solve your problem! 

It's been mentioned dozens of times already by many people, including there:

I doubt it was necessary to make a thread about it, but oh well.

As there is already a topic, there is no reason in keeping this topic open. On-top of that, it's not an LSPDFR issue so it has been moved.

If you feel this topic has been closed in error, please report this post.