How to repeatably darken pennies across the whole spectrum from bright to black for art project?

Q. There are countless web pages that talk about how to clean and brighten pennies, but I want to find a controlled and reliable way to darken them to a specific shade, all the way to black. I've been scouring the web (and this site) and have come up with some distinct possibilities, but would like some experienced input as to the easiest, safest, most reliable, and/or most thorough way to do this. Here are some possibilities I've come across:
* Submersion in some type of solution -- perhaps zinc oxide ⇦ on eBay or Amazon [affil link] ?
* Chemical rub -- such as liver of sulfur ⇦ on eBay or Amazon [affil link]
* Speeding up oxidation -- of which I don't know how.
* Electroplating or galvanization

My primary goal is to produce a spectrum of pennies of varying darknesses between its original shiny copper color all the way to a reasonably dark black, and any shade in between.

My secondary goal is to use zinc-plated pennies to produce a spectrum of pennies of varying darknesses between their shiny silver color all the way to a reasonably dark black, and any shade in between.

This is for an art project.
Any suggestions would be greatly appreciated.