I obtained this in the "Sandbox" mode (unlimited money, all skills available, and no customer cars), if you wish to obtain this trophy outside of your story playthrough.
You can still follow this guide if you wish to do this within your story/normal mode, though you need to have the paint booth unlocked by having at least 30 skill points/ranking/whatever you like to call it.
It also costs $1,000 to paint a car.
To get to Sandbox:
--> main menu
--> profiles
--> create new in an empty slot
--> select the new profile
--> choose "Sandbox" mode
Buy a car from the barn, junkyard, auction or salon, and get back to your garage.
Move it to the Paint Shop, walk forward to a big, round, dark blue object, toggle
to "Paint Car", and select to paint factory color by
.
To move to the Paint Shop:
-->
when next to the car
--> toggle
to "Move Car" (orange icon)
--> press
on "Move Car"
--> toggle
to "Paint Shop" (red icon)
You can also get this trophy from dealing with a customer car, who wish to repaint their car to factory color. You do not need to buy a car specifically for this trophy.
I also obtained "Appearance Matters" for just painting a car, at the same time.