MechWarrior 5: Mercenaries

At the beginning, after the tutorial, you'll be asked by Jahad to repair the Centurion. Instead of selecting repair, select paint mech and choose a different design. Press apply to unlock the achievement.

I didn't want to do the tutorial, so I choose to skip it after starting a new game. When you start follow the way point and talk to the person next to the table. Go down the stairs to the lower level, talk the guy at the way point. Then interact with the console behind him. highlight the first mech, if it isn't already, and hit A. select paint Mech. Make some changes and hit X (Apply) and achievement pops.