I'm trying to make the lights on the Ram 3500 from this pack red and blue instead of just blue. I tried editing the file by going into Paint.net and putting red over the blue, but nothing changed after replacing the original file. Any help appreciated.

It's because the model's lights are mapped to the blue texture only. Whereas some authors choose to map them to both the red and blue and just change the texture. Only fix would be for the author to change it.