Hello,
I added some orange to the black Polymethcolours from Beufa, and it does look more neutral, however, above a certain ratio of orange, the color turns more towards violet. So perhaps some yellow instead of orange, or a mixture of yellow + orange could bring the black much closer to neutral from it's original bluish tint.
On unpolished parts with only the standard sandblasting, the black looks pretty close. On polished parts, the tint is much more visible because of it's strange frosty surface, which makes the color lighter, or more transparent, revealing the tint more clearly. Also, the polish in black doesn't look very good because all the pock marks become very visible, kind of like a porous stone surface. Black looks much better on unpolished parts.
The solution to black polish is probably to clear coat it, covering up the strange frosty texture, and at the same time making it look darker. I have also heard that another company sand blasts their polished parts before dyeing, just a light touch, probably to get rid of the frosty surface.
In terms of concentration, I tried using 10, 15, and 20g of polymeth in 1.5 L I think 15 and 20 were exactly the same, so after a certain concentration there isn't much difference. Actually, with the iDye Poly, adding the entire 14g packet to 1.5L of water caused the powder to clump and cake together. I'm not sure if maybe there wasn't enough of the intensifier liquid that was included in the package, but as I don't have extra, I can't test it.
So to summarize what I've found out so far:
Unpolished parts look best for black
Polished parts look strange when black, additional coating may help
Polished parts look lighter and tint is more visible
Black dye never turns out black, you should mix a complimentary color to neutralize the tint
Probably the same with every other color, as the polyamide doesn't react with the dyes as the manufacturer intended
Grey seems to turn out violet and is too dark
Acid dye doesn't seem to work. It did work the first time I tried it, but repeats were unsuccessful and I don't know why
Dispersion dye seems to be the best
Jacquard iDye Poly is a type of Dispersion dye, but it's not working so well, and stains a lot
Beufa Polymethcolours is also a Dispersion dye, and it works pretty well, but colors need to be mixed to get the right tint
Polymethcolours has minimal staining
Beufa's Lavegal cleaning solution works well at removing the excess dye in conjunction with an ultrasonic cleaner
Ultrasonic cleaner immediately releases excess dye. If you tried just heat and a stick to stir, it takes a while before the dye gets out
Repeat cleaning seems to always remove some dye, but each consecutive bath has less dye in it
Not sure if it's 100% possible to make polyamide completely stain free
I haven't tried any shapeways dyed parts in an ultrasonic cleaner so I don't know if that will release dye also
The people at Beufa were helpful and I am maybe 90% sure I will buy the black dye with a bit of orange and yellow from them with their intensifier and cleaning solutions. It is a bit expensive though, although similarly priced to the iDye Poly, not as cheap as acid dye though, about 2.5x more. They also might only sell to companies, or if they do sell to private persons, there is a minimum order quantity. You could possibly ask them for samples to test.
Someone who sold me Jacquard packets recommended trying DuPont dyes but it is in liquid form meant for paint / steam dyeing. There are some instructions floating around on how to dilute it for stove top immersion, but I think I've tried too many options already and I'm not really wanting to test another one, unless someone else wants to try it?