Is nitric acid the only passivation choice?
Citric acid passivation is also available at H&W. Citric provides a coating as good or better than the nitric acid solution.
Can you see a passivation coating? What color is it?
No, passivation is a clear coating
How thick is a passivation coating?
Almost negligible, invisible and only a few molecules thick.
Why is passivation even needed when using stainless steel that is already corrosion resistant?
Stainless steel is indeed corrosion resistant, but the surface is contaminated by machining, fabricating and general handling. The natural protective layer has been compromised.