@OrcaellaSaurusIDPaint.net is like adobe photoshop. I recommend Gimp2 as well its a free program and does a good amount of the things photoshop does. Pretty sure it does more editing then paint.net. You will need to find some plugins called NormalMap, insanebump and bumpmap to make normal maps for any texture that you make. You will also need the dds plugin so you can load dds textures which is what gtav uses.
@Theoasterix yes you have to change the modkit id in both carcols.meta and carvariations.meta files. Now if you really want all your addon cars to have all their ids working then do what i did and find carcols.ymt under x64a.rpf and search for <modkit> under that section delete all the ids higher then 0. You want to keep the ids that have 0 in them. Once you delete all the ids that are 1 or higher the vanilla modkits wont work anymore but the modkit ids of all your addon cars will now work. Along with any headlights and lights that werent working before on your addon cars. MAKE sure to make a backup copy of the original carcols.ymt file.
@MrCool you need to use a program called gimp2 or photoshop. Gimp2 is a free texture editor its like photoshop with less plugins. You will need to extract the interior textures from the car you want to change the color of the interior. Then open the texture file with gimp2 hit colorize at the top and change the color by moving the slider. Resave your file with the new color. I suggest saving it as a dds file with either dxt1 or dxt5 compression and select generate mipmaps .