Some vehicles in gta5 cannot change default trim color, for example my favourite suv cavalcade, but I really want to change it to brown. My friend told me I can do that by changing a number in carcols or carvariations but I have no idea how to do it. Anyone know?

I downloaded a mod called "coquette gte" this week and it is made by ryowatanabe201
This car has 4 different liveries but I cannot use them. No matter if I use simple native trainer or any other trainers the livery cannot be seen.
I want to know if anyone have the same problem with me? let me know!

In the SP game，if your character purchase a weapon then you can have it forever，no matter what you do they will always be there. 

But if you want to have online weapons in offline game，then you need a trainer to spawn them.  once you have your weapon，save them in the trainer，next time you start your game，load them from the trainer again to get them back.

To use addon mods，you need to install:  packfile limit adjuster，heap limit adjuster. 

gameconfig is also important but you need to choose right one.  I have 200 vehicles and I am using 3X traffic  3X peds.  which means this gameconfig can handle this amount of addon vehicles.  try to find the one you can use based on how many addons you have.

  This can be caused by using wrong gameconfig or the vehicle mod have some problems. 

   I have 200 cars installed and I am using 3X traffic  3X PEDs.  But if I use the other config like 2X traffic 2X peds then my game can crash after loading.  see if you have the right gameconfig.

If you install your car mods correctly and they are addons， find and install these plugins:

 packfile limit adjuster
 heap limit adjuster
 gameconfig

read the instructions and install them all，then see the problem is solved.

Usually，if you want to play DLC vehicles， the best way to do is spawn them manually by trainer.  If you want them automatically spawn on the street，it is ok but make sure related mod are the latest version and you installed everything correctly. 
For example，I had IVPack installed before，they can spawn IV vehicles and some DLC vehicles on the street. But this mod is currently outdated，so if you install them and the vehicles spawn on the street crashed my game.

Now I can only spawn them with trainer and disable spawn on the street and wait until a new update is available.

   I  had about 200 addons before，including addon vehicles and 15 interiors.  but when I enable mp maps my game crash.  finally I found  some of the interiors cannot compatible with my map，even they are addons. 
 I am not sure if your tree mods are addon? or maybe you enable the mp map at the same time? check them.  If it is then try to stop the tree mod and check your game again:)

You can use a trainer to spawn what you need. I am using simple native trainer now. it can spawn all online content，like maps vehicles and weapons. highly recommended!