It should work with all other cars. But spec and fresnel textures doesn't work then. So chrome parts are not possible (at least I couldn't make them work).
What you need to do:
1) Add alpha layer for you livery file. Paint places where you want matte effect black and glossy white. And save it as DXT5 interoplated alpha.
2) I suggest you make copy of my livery_merc_matte.mtx, rename it and then you can edit different values there for your liking, play around a little.
3) replace paint materials in *.rcf file. Easiest would be to install livery with Custom Livery tool and then just replace NEWMATERIAL="..." /> with the one you want NEWMATERIAL="vehicles\_generic_materials\livery_merc_matte.mtx" />.
In case on Z4 - Custom Livery tool adds these lines
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint" NEWMATERIAL="vehicles\_generic_materials\livery_generic_effects.mtx" />
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint_skin" NEWMATERIAL="vehicles\_generic_materials\livery_generic_effects_autoskin.mtx" />
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint_cpit"
NEWMATERIAL="vehicles\_generic_materials\livery_generic_effects_autocpit.mtx" />
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint_cpit_skin" NEWMATERIAL="vehicles\_generic_materials\livery_generic_effects_autoskin_autocpit.mtx" />
And you just replace NEWMATERIALS with your material file name in there. So it would look like this
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint" NEWMATERIAL="vehicles\_generic_materials\livery_merc_matte.mtx" />
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint_skin" NEWMATERIAL="vehicles\_generic_materials\livery_merc_matte.mtx" />
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint_cpit" NEWMATERIAL="vehicles\_generic_materials\livery_merc_matte.mtx" />
<REPLACE MATERIAL="vehicles\bmw_z4_gt3\bmw_z4_gt12_paint_cpit_skin" NEWMATERIAL="vehicles\_generic_materials\livery_merc_matte.mtx" />
I know that all those cpit and autoskin material files are different but I haven't got around recreating them for matte paint. Maybe will look into them more deep but meanwhile I haven't noticed anything wrong by replacing them with same material file.
Also you can check here
http://forum.projectcarsgame.com/sh...tallic-Paint&p=1269678&viewfull=1#post1269678 , added files and metallic paint there