Maybe they got some different peoples wheels mixed up at the service shop during a tyre rotation. But if the odd one out wheel is not an official colour then that would be unlikely.
As suggested the wheel has probably been repainted from curbing damage, especially being the front left wheel. From my experience with wheels that get refurbished you can usually spot them. Firstly the tyre sidewall may just be a little damaged from when they sand the rim back and when repainting some fine over spray makes its way in the spaces between the spokes. This leaves a rough to the touch finish. Of course if it has new tyres there will be no tyre damage and if the whole wheel has been repainted then there should not be over spray.
Looks like you will have to get it repainted in hopefully, the correct colour. Try the car dealer you bought it from, see if they will rectify it, or at least some agreement over cost. I would think they made good money on the car they sold so should be 'happy' to keep you happy with a few hundred spent on a repaint. I'm not too sure on the laws in Australia but you may be able to pull some levers, although I always hate the thought of doing that. See if you can find any pictures of the car before you bought it or when they listed it to see if the wheels were a different colour then.