Is it possible to Dye parts which are already plated.

A. Screws with heads can be color-coded by PAINTING; for set screws you are stuck with plating.
Unfortunately, the dyes do not adhere to finished parts.
My suggestion would be to make colored batches in advance (examples: silver zinc, yellow chromate zinc, black chromate zinc and green dye / red dye over zinc).
unfortunately, all these finishes must be applied during the electroplating process, so you cannot go back and decide that your bright zinc screw should suddenly become red -- the dye will not stick.
In fact, the colored dyes (not to be confused with chromates which give permanent color) already adhere poorly to the chromate finishes even when wet (i.e. when the dye is applied immediately following the chromate coating).

A. Hi Alan,

I have successfully dyed both zinc and cadmium that have either previously been dyed or chromate passivated.

First thing is to leach the colour, this is done by immersion in 10% caustic at room temperature until all of the colour has dispersed. The length of time will depend on how old the parts are, the older the part, the longer it takes. There does seem to be an age where it doesn't matter how long you soak the colour will not disperse, obviously you can't do a lot with them. Rinse the parts very well in distilled or de-mineralised water to ensure all of the caustic is removed.

Next is to immerse in the dye of your choice. Dyeing time will depend on what dye you are using, if you use it heated or at room temperature and what concentration of dye you are using.

Now for the caveat...the dye doesn't always take first time, sometimes it takes 2 or 3 immersions. I have usually taken 1 or 2 bits to the lab to play with first to see what needs to be done to get the dye to take.