Today most of the manufacturers offer a decent choice of out of the box liveries, but judging by an ever busy works queue, there is still plenty of demand for customization!
Locomotive resprays require a lot of work and are very labour intensive. The existing finish is completely removed, back to bare plastic. The surface is prepared, primed, and sanded/keyed for the application of livery colours.
As with most things in life, good preparation, care and attention to detail will pay rewards with the finished article. The following pictures show a Heljan O gauge class 45, modified and repainted into a class 46.