Trading Paint Achievement
Earn 20 medals in Multiplayer races.
You can join public matches and earn medals if you place high enough. Alternately, if you have one partner to boost this with, you can do it all in a private lobby as detailed below:
From the main Forza screen, choose Multiplayer, press to create a private match and to confirm. Once the vehicle selection screen opens, choose a car that is rated B or below. At this point you will be in the lobby. Invite your boosting partner in to the lobby and ensure he picks a vehicle rated B or below, and then press to change the options to match these choices:
Game Type: Drag
Track: 8-player 1/4-mile drag
Max Players: 8
Max Drivatars: 6
Drivatar Difficulty: New Racer
Car Class: Any
Under Advanced Rules, change the End of Race Timer to 5 seconds.
When you go back to the lobby, all the Drivatars will have a class of car that matches yours. Both you and your boosting partner will need to press to go back into the settings and choose to change your vehicle. Opt to rent a car and choose the 2013 Chevrolet #1 Andretti Autosport Dallara DW 12, which is the fastest rentable car. You will both end up back in the lobby with your fast cars racing against the lower rated vehicles the Drivatars are using, making the drag race basically an automatic gold and silver for the two of you, ending in about 13 seconds. Because the car is rented, you will earn no money or XP, which skips those screens and significantly speeds up the process. If you press at each Post-Race screen, it will take approximately 4-5 minutes to complete the 10 heats, at which point you will be back in the lobby. Press to restart the process.
Note that you can change the length to 1 mile instead of a quarter mile if you want to work on the Affinity level for a manufacturer at the same time, and then instead of renting a car pick a fast vehicle from the manufacturer you are leveling up. It adds about five minutes to the amount of time boosting each set of 10 heats.
The achievement will unlock at the end of the heat in which you get your twentieth medal.