To unlock this trophy, you'll have to defeat the 6 "Top Dog" bosses. The best way to defeat these bosses, is to evade once they charge at you, give them a quick blast with the shotgun and then run up to them and perform a "Shiv Finisher" on them. Repeat this 4 or 5 times and once you bring them down to a low enough health, a prompt to press "X" will occur. This will finish the fight. Make sure you've stocked up on ammo and upgraded Max's health and ammo/used Griffa tokens to get Max to his best. Once you've defeated a Top Dog, you'll gain their paint to use as your car body colour.
The bosses you'll need to defeat, where they're located and what body colour you'll get:
Gaspa Grope - Blackmaws Region, Jeet's Territory - Black Tar
Stump Grinder - Colossus Region, Jeet's Territory - Racing Paint
Gut Noose - Cadavanaugh Region, Gutgash's Territory - Primer (also gives 50% fire resistance, unique to this colour)
Rim Jobbie - Chalkies Region, Gutgash's Territory - Raw Steel
Pig 'n' Sticker - Rot 'n' Rusties Region, Pink Eye's Territory - Camouflage
Cock Locka - Grandrise Region, Pink Eye's Territory - Gold Tar