It does seem pretty stupid when so many cars have alloy wheels and quite a variation in colour, that car makers do not support us with providing suitable touch-up paint. In my Ford days I used a very old Ford silver, think that worked for VX and maybe VW but it is solid now! Yes very very annoying trying to match wheel colour! I bought a set of used Audi 18" alloys for winter use, one looked like it had been eaten by a bear(?), so I ended up getting a single new one from ebay, went to get the 3-off used ones refinished, but was advised by powder coating place to include the brand new one so that they matched - that hurt!
I'm sure that someone on a VAG forum mentioned "German car wheel paint" - but that assumes they are all the same!