The easiest way to paint plastics at home is scotch brite panels so there dull all over, spray 2 coats off plastic primer over top, let it dry, then spray your colour over the top,
some auto paint shops can mix most car colours up and put it in a pressure pack can, if you do this you can get them to add some flex aid to the colour befor they put it in pressure can, this will make the paint flexible and not brittle, as that is the main problem with paint cracking over plastic,