Both of these choices have their advantages and disadvantages. If you choose to paint them by first removing them, it may be a much easier way of doing the job, as you can paint them on the ground. However, you will need a very large space to lay them out to dry, which could take several hours. You will also need to paint the metal securing strips to match your tiles. However, if you choose to paint them while they are in position, it may be difficult to paint so high over your head.