Give any GTA V car its own liveries, even cars that never had a livery slot. Paint right on the car's livery template like in Photoshop: colours, gradients, images, text, with the car updating live in 3D. Then download it as a new add-on car, or as a livery for the original car.
In this video I open the T20, paint a chameleon gradient, add an image and some text, and download it as a new add-on car (mic T20), then test it in game. At the end I show how to add it as a livery to the original T20 instead.