To mix a dye, you must do it on the actual Dye screen. There are 4 slots for this, which also limit it. Anet is assuming you're buying the dye for you instead of others. I know, I used to love buying pre-made gold or white dyes, but it is over now. I made such a living selling blood-red T_T
You can, you just have to get all the dyes yourself and then mix them. You just do One dye at a time, then add more. Just double click a dye, click your armor, and in the dye screen add more dyes for the combo you want..
For probably a year it's been like this. You can't pre-mix dyes anymore. (and the armors themselves were changed, so colors affect them differently now)