This can be unlocked once you have acquired your first car in Career
From the Career menu select Paint
now you can either open the Paint Car
menu (To paint different parts of the car), the Apply Decals
menu (To add various decals to the car), the Tint Windows
menu or the Create Vinyl Group
, no matter which you choose once you have made at least one change (Changed a car part's colour, added a decal, tinted the windows or created a vinyl group) press
to save the design and the achievement will pop.
Here's a video demonstration with changing paint.