Some of the robots can be customized with special skins, also known as paint jobs. There are two types of paint jobs: standard and legendary. While standard paint jobs change robots' appearance only, legendary also give them an additional characteristics bonus.
In order to apply a paint job to your robot:
Choose a robot;
Choose 'Paint job' at the bottom of the screen. Note, that if there are no paint jobs available for the chosen robot, you will see only '3D view' option;
Choose a paint job you like and tap 'Buy paint job'.
A paint job is transferable: it can be unequipped and applied to another identical robot. You can sell paint jobs you don't need like in the same way as you'd sell any other equipment. When a robot is sold, its paint job will be sent to your inventory.