I've yet to master getting that early greenish look and keep the iridescent swirls.
I can do it by making the solution super weak, but it comes out a good bit greyer. Other attempts either end up grey or matte olive drab, which although it looks neat, doesn't look totally right.
I really need to keep my one green batch as it is now, and start playing with the ph of another batch, or trying different presoaks. It'll be funny, citric acid presoak will work great for one chromate, and have zero effect with another. Phosphoric acid will work great with one, and be highly variable with another, etc. Same thing with different cleaners that are used to prep the castings before etching. They have a dramatic effect on the end product.
Part of it is the yellow I have can be dried at higher temps which aid the rainbow effect, but if I do that with my greenish chromate it ends up killing it, as that chem is considerably less heat tolerant.
Still a work in progress, but I've officially been doing this for 14 months now, roughly 120-125 carbs restored.
I now have about 12 different chromates, (maybe more) and haven't really figured them all out yet, just need more time experimenting with different presoaks and drying techniques, but it's hard when buying a batch often costs $600.
Either way, just updating this thread since my last post was in June :p
Luckily, I had a customer send some carbs from Joe Bunetic, and I've had a few others sent to me from other restorers as most of those folks apparently do not test out the carbs on an engine. It's been nice to compare their results and try to figure on how they did what to get what effect.
My actual zinc plate has gotten considerably nicer on the steel hardware. I gave up using a "commercially prepped" formula and went ahead and made my own chloride solution. At least this way I know what is in it.
Herb Geduld wrote a great book on zinc plating, a worthy read for anyone into learning this. Sadly it's a very expensive book.
