My opinion is that all bathrooms can be of different finished but the same fiish should probably be used throught each bathroom. I have black in one bathroom and Kohler Purist gold in another...look great. I would try to match dooknows to the fiish although this might be difficult with a gold finish. You need to get a good retailer or showroom. Be aware that some retailers will almost automatically give you 25% off retail price; this was the caase with two showrooms I worked with this week in NYC. Did not even have to ask but if you are a new client, you might have to...are you working with designer or arachitect who can pass on the trade discount to you? Asks! It makes a big difference in final pricing. Not sure if the discount would be offered at big box stores like HD or Lowes.

Black doorknobs seem to be a classic neutral in houses (especially older houses) - something that people don't really even see. Certainly you can have a black doorknob and a different metal in the other fixtures, as well as different metals in different baths. It really depends on how important it is to you to have everything match.