JordanMunk

One helpful thing to learn about is to search your question on google, you might find the answer 🙂
 
If you're referring to the lightbar lights...
 
 
 
For the texture color of the lightbar itself (not the lights it emits), you can find them with OpenIV. Just search the model of your car on there, open the .ytd file up, and look at the textures on the bottom right part that lists what textures your vehicle uses (and it will direct you to them). If you want to edit textures, I strongly suggest making a mods folder for that, if you hadn't already (OpenIV will prompt you to do that if you try editing).
 
Though, I think the actual lights are controlled somewhere else, I'm not sure about changing those.
 
As for headlight color, iirc an update supports changing headlight color so you can change them with a trainer.
 

It depends on what edits you want to make for the model. If you want just texture edits, those are simple, as OpenIV can open up .yft's (texture files for models) and export/import them (and you can preview their respective ytd's -model files- to see what it looks like) and you can open the .dds file in whatever image editing program you like.
 
If you want to do full-out model replacements, you need a 3D modeling program that has support for GTA V formats, such as 3DS Max 2016 with GIMS installed. Keep in mind, I haven't done a vehicle mod for GTA V, I've done only ped mods so I don't know how similar the two processes are.