Hi Christian,
Sorry I didn't get to you sooner but it's obvious that you eventually worked it out. It's amazing what a bit of age and rust can do to make things harder than they seem.
we're still hoping that Mr Jones can get his paint supplier to come up with a formula as that will make life much easier.
I must admit I've had quite a few paint suppliers only too happy to colour match Hammertone paint here in Adelaide. Some suppliers just want your money and aren't interested in doing anything for it as I've found in the past.
Cheers,
BB