Here is a place where I post answers to common bismuth questions.
Q: Can the color come off my bismuth pieces?
A: The color is superficial. It can be scratched off with an abrasive. It can also be chemically removed with strong acids. Heat generally won’t affect the color until you approach the melting point.
Q: Can/should I clear coat my bismuth?
A: Lacquers and epoxy tend to either physically or chemically interfere with the superficial oxidation colors and will likely remove much of the light interactive color, leaving your piece looking dull. I would leave bismuth as is, unless you have a sure-fire method that works for your needs.