I've done a few sets of alloys in the last year or so - my main tip is take plenty of time when you are doing them, cos if you want them to look good its going to take you a while.
In terms of matching paint colours, as the earlier messages have suggested, its not easy, so you are best filing down the kerb marks, lightly sanding the whole wheel to give a 'key' for the primer to stick to, then prime and paint, followed by lacquer.
If its a Rover / MG wheel set, the colour for most (if not all?) alloys is MNH Silver Sparkle.