I'm yet to see any holden, or just about any car for that matter where plastic matches metal with metallic colours, and even a lot of solid colours. Metallic flake and pearls sit differently on plastic panels compared with steel and even aluminium because of a variation in static charge. The different coloured substrates and/or primers for each surface can affect the look as well, particularly if base coats are applied thinly (OE paint finishes). It takes A LOT of experience and often playing around with base colours to get them close. Something that will never happen with an OE finish.
Also, as said, they are painted off-car - and not by holden - so while the colour code will be the same, the batch will almost certainly be different, meaning there's a good chance there for variance as well.