Hello!
I ran into a bug: when choosing brake painting, you can not use the color already applied to the car (the color in the cache).
This does not happen with all shades.
Please see if something can be done.
Sincerely, your player of the standard version.
Demo: Forza Horizon 5: it is impossible to show off with painting brakes (whining) - YouTube
Are these manufacturer colours, because manufacturer colours are not just one colour, they are many different colours all at the same time.
I think the OP is referring to not being able to automatically apply a colour that’s just been used in the vinyl or livery editors. The colour will appear in the ‘Paint Car’ section but you have to press X to find the HSB values, then go to the ‘Normal Colours’ palette and input those values. Now the game will recognise it as previously used and will apply it to the wheels (or mirrors, wings etc.) if selected.
To the OP, it’s been this way for as long as I can remember, best to work around it and move on as it won’t be getting ‘fixed’ anytime soon 
This is Brake painting, it’s new. I’ve never had that problem with using old colours.
I don’t know for sure if it was possible to paint these parts of the disc brake in early versions
(I’m a beginner in the series), but in this case none of the colors I used earlier was the “manufacturer’s color”…
Painting callipers is new but the principle is the same.
Well best to just use the separate colour palette rather than the saved colours.