By far shadow chrome is the most difficult colour of wheel to perform alloy wheel repair on.
Firstly the factory process for a shadow chrome colour varies from factory to factory , but basically the wheel is powder coated to a gloss black and then lightly mist coated with a silver/chrome paint which is translucent and allows for the black to show through which produces that dark chrome effect, finally the wheel is coated with a clear coat.
When repairing shadow chrome wheels we sometimes recommend to respray all 4 or 5 wheels on the car ( including the spare in some cases ), this ensures a uniform colour for each corner and more importantly it actually works out cheaper than say repairing only 2 wheels and spending the time to match a shadow chrome finish.
Having said that there are a few methods we are experimenting with in order to get great results for shadow chrome matching.
Unfortunately we are keeping this a trade secret and will not be revealing anything further on this blog regarding the process.
You could start with a good dark primer I suppose and then work your way from there.