I have a variety of thoughts on how the dyeing process currently works in-game--my biggest confusion was finding that copper is not an in-game mordant, and (as far as I can tell ?) the mordants that you can use all seem to require a pulverizer.
For background, I dye textiles pretty often IRL and have some knowledge in that area. And I think the real life system of dyeing is a lot easier and less complicated than the in-game system is, and so in this case it would be very cool to go for realism.
So here's my suggestions:
1) Take the pulverizers out of dyemaking. The current mordants (alum and cassiterite especially) do not need to be a powder. I'm honestly not sure about chromite--I've only seen it in powder form IRL. However, I don't think chromite is even a necessary part of dyemaking in-game.
2) Add copper and iron as mordants. These are fairly common mordants in both historic and modern dyeing, and quite easy to use.
3) Add a metal dyepot method, where the player makes a mold for a dyepot (could just look like a cooking pot), fills it with molten copper, tin, or iron, and then cooks the textiles and dyestuff together in the pot. No additional mordant needed.
The player would always need more dyestuff, but not more mordant. More dye colors would be available as the player progresses through the eras (more on that in a second), and it would be a simple process. Take your metal dyepot, put it on a fire (on the lowest temperature you can get, ideally), add the dyestuff and the textile, cook it for a few minutes, and you're done!
4) Mordants affect dye colors !!! In real life, I mean. And they should in-game, too. An easy way to implement this would be to change the tint, tone, or shade of the color (essentially, it would modify, darken, or brighten it). This way, you could stick with the current dye materials, but a whole new range of colors would be added simply by using one metal dyepot or another.
A copper dyepot makes the dye more green, so blue turns into blue-green, yellow turns green, etc. Some shades aren't affected by the green (pink, for instead, only ever turns out pink when I use copper mordant). This would make the copper dyepot somewhat unpredictable and wouldn't allow for some colors (yellow, for example), so players would have incentive to keep progressing.
A tin dyepot brightens the dye, unlocking bright yellows, brilliant reds, emerald greens, etc
An iron dyepot saddens the dye, unlocking darker shades such as navy blue, maroon, dark gray, deep brown, olive green, etc
5) I was pleased that woad doesn't require mordant, but I'd love to see walnuts added as another dye material, one which also doesn't require a mordant. Walnut hulls (the green part around the shell) make a fantastic dark brown or black dye.
Dyes which don't require mordants could still be cooked in dyepots, but type of metal simply wouldn't affect it.
6) No more dyestuff soaking. I cook all my dyestuff to get a liquid dye, which I then put in a dyepot with textiles. Soaking dyestuff takes a long time, and often doesn't extract all of the color.
The only exception I can think of in the list of dye materials in-game is woad, which does require soaking. If indigo is added in-game, it should have a similar treatment. The way woad dye is made and used in game is very nearly perfect--I only think it should require the barrel to be sealed for a week before you get dye out of it. And then the player just combines the dye and the cloth in their crafting grid, no heat or cooking necessary.
It would mean that there are two different methods to dye textiles with (metal dyepots + cooking the dye and textile, vs. soaking plants in a barrel for a week + adding textile to it), but there's relatively few plants that can be used with the second method, so overall metal dyepots would be the default.
And as I said, this would add an element of progression to dyeing, as certain metals only become available in large quantities later in game. The current dye system requires barrels (so, copper age, but you only have one color you can really do), and then pulverizers (late game). No real progression between the two, IMO.
Please let me know what you think !