One of the things I loved about th e old Color Picker mod was the ability to edit the alpha channel in trains and wagons. It greatly increases the range of colours you can use, and makes it possible to use purer colours, rather than the somewhat washed out versions we currently have access to in the current Vanilla game.

Would it be possible to add that to Wagon Colors??

1) Adding alpha channel would require adding a gui, which is outside the scope of this mod.
2) Wagon Colors has been superseded by Fluid Wagon Color Mask, yeahtoast made a much better looking color mask.