I'm not sure why it's literally impossible to make dyes in primary colours, but I think this needs to be changed. It's fine if creating primary colours is hard, and takes a lot of time, skill and materials, but it seems silly to make it completely undoable. As things currently stand, there are two factors that prevent us making primary colours. The first is that there is a limit to how high the RGH values go. I think I heard somewhere it was just above 200? Since for any primary colour you need either red, green or blue to be 255, 200 makes it impossible. The second factor is that for some reason, all of the different coloured ingredients don't contribute only their own colour. For example, a cochineal is for red, but if you make red dye with a cochineal, the dye still has green and blue values. Either we should remove this, or make it possible to strip certain colour values from a dye, as you're not going to get primary colours without two of the RGB values being 0.
I wouldn't even mind if the whole system was made harder to make up for these changes, but I'd really like to be able to make primary colours. We'll also need woad planters, as mentioned in another post, as woads are the only materials that are practically not available even to people with high skills.