Iray materials question for fingernail (and beta freebie)

StratDragon

I'm trying to make a nail polish overlay for Genesis 8 Female that adds an displacement overlay to add realism.
the issue I'm running into is when I use light colors as the metallic material surface they look fine, but if I use darker colors they fade closer to gray.
I would like to avoid using textures for color swatches if it's possible

the test files can be found here and require base genesis 8 female textures.

https://www.deviantart.com/stratdragon/art/Gen8FPreManiPedi-857381946