Visual customization can be accessed through your garage > visual customization (spanner icon). Here, there are many different areas to which you can customize. Certain cars will have different areas to customize than others.
To unlock these areas, you need to complete a specific criteria. You can view these by hovering over the part you want to customize.
General list of what you can customize on your car:
•Hood
•Grille (certain cars)
•Front Bumper
•Splitter (certain cars)
•Headlights
•Front Canards
•Windows
•Front Wheels
•Brake Discs
•Brake Calipers
•Rims
•Tyres
•Front Fenders
•Wing Mirrors
•Side Skirts
•Rear Fenders
•Rear Wheels
•Spoiler
•Tail Lights
•License Plate
•Rear Bumper
•Diffuser (certain cars)
•Exhaust
Trunk lid
Sound system
Roof (certain cars)
I had all of them unlocked but no achievement. I then brought a Land Rover Defender 110 (Offroad) and found the missing customizable point on the roof.