Hi Glenn,
Colorbond finish is an enamel type paint, ie oil base/non water based solvent in paint. So you are not repainting on steel but simply repainting over an enamel based finish.
Main criteria in repainting over enamel and most other painted finishes is to prepare the surface for recoating. Normally this means cleaning dirt and dust off the surface, and if a glossy finish, light sanding to break the gloss and give a finely sanded (grainy/scratched surface) which paint adheres to more readily. If the surface is in good condition (no flaky paint), using sugar soap to wash the surface may be sufficient preparation prior to painting.
cheers