In order to turn your armor invisible add the word "hidden" on an anvil.
It does not matter where you add the word. You can leave it all lowercase, capitalize or aLtErNaTe LiKe ThE mEmE bOi YoU aRe.
You can even add symbols, other words or even omit empty spaces.
It will also support special formatting (color, bold, italic, etc) if you're using /give

Valid examples:

Hidden Diamond Chestplate
hidden Diamond Chestplate
hiddenDiamond Chestplate
My hidden Diamond Chestplate
[Hidden] Diamond Chestplate
Diamond Chestplate Hidden
hidden
