/signed to doing something about the dyes and how they work. Dye remover should be like a bleach and remove all color pigments making the item white. The dyes are then applied and the color dye used actually shows up. To make a darker color you mix the color dye with black just like real life and the ability to get different shades. I mixed 2 red with 1 black, because the red alone was too pinkish and the 1 red and 1 black was too dark, and got mixed dye that, when applied, turned out to look the same as if I used just one red. What a waste of money buying the black and other red. Applying multiple coats of the same color would give a deeper more rich outcome of that color to the point where (after 4 or 5) the color can't get anymore rich. If you have a piece of armor that is blue and you apply red then the piece of armor should turn purple, the red shouldn't replace the existing color. To replace the color, dye remover should be used before applying the new color. As for mixing, the last I checked, 500ml combined with 500ml gives you 1L not 500ml. One dye plus one dye should yeild 2 mix dyes.
A preview at the dye trader would be nice too. Nothing fancy just when you talk to her you chose the piece of armor you want to see dyed and select the dye(s) then press the preview button to see the outcome and price. Then you can either hit accept to make the changes or decline to go back and try different colors. The change in color would only exist in the preview mode unless you press accept then it's perminant. Allowing others to see the preview if they look your way would be good too. Can you picture entering a town, head for the dye trader and see a dozen people standing around her changing colors like the northern lights.
Dye is used to change the colors of fabrics including wood and skin not metal. To change the color of metal portions of your armor then you should be able to do so at the armor crafter and see the weapon crafter for the metal portions of your weapon. Weapons like the storm bows and chaos axes are unique. The storm bow is the color of an electric arc and has what looks like electricity flowing all around it and therefore shouldn't be able to change color. The chaos axe looks like it too is made up of energy and therefore shouldn't be able to change in color however it could drop as a different color at random. They could make it so those weapons can have their color change but it would require seeing a weapon crafter in the FoW. The eternal bow is the color of ecto and shouldn't be able to change color (not by conventional means) and may be done but require seeing a weapon crafter in the UW.