On the 1968 Ford Mustang GT 2+2 Fastback you can’t paint the wheels with normal paint. Only with the HW
That is not true. The particular wheels you used do have an issue with various different kinds of paints (including HW you incorrectly listed as functioning), but the issue lies with you using colours from… the Livery Editor. Yes, when you use a colour to apply for any vinyl, it then appears available in the painting menu, but appears as some sort of invalid or textureless colour.
I finally have some screenshots
Here you can see me using Normal colours on stock wheels, they work just fine
Here, these Halibrand wheels are also painted using a standard colour
Now this one is a bit interesting, because this light blue you can see on my recent colours list doesn’t appear as light blue at all, and instead is some sort of missing colour placeholder. This happens when you apply colours to vinyls in the Livery Editor. These colours will always appear on your “recent colours” list but will never be usable
And here I’m using the same colour on the same Halibrand wheels, and of course they don’t work here either