Hi Tom — That's a nice bicycle you have there and I love the mermaid paint. I have the same on my Appaloosa.
For bar tape and other accents I aim for harmony. Sometimes that involves contrast, sometimes it requires neutrals. I think that Dave's khaki Newbaum's is a great pick for the bar tape on his Platypus. Echoed by the coffee brown walls of the endurance casing (or endurance plus?) tires. I'd wager it looks even better in person as I've found my cameras don't quite accurately render the mermaid paint color. It's lighter and more subtle, further from Atlantis green than suggested by many pictures.
When I first built up my Appaloosa I was aiming for harmony but looking back it seems quite "matchey-matchey." Gray tape and saddlebag. Maroon tape with amber shellac gets a deep red-brown that's close to the leather saddle and leather on the saddle bag. I have to say that looking at it now the black rear mech, front mech and cable hanger make me want to scream out loud. I've since swapped them out.
I would recommend light gray Newbaum's with clear shellac for a Rivendell mermaid bike. I think given your black saddle, crank, bars, gray grips, the light gray would work well with the rest of your build.
This is merely my opinion but I very much dislike cable housing and tape matched to the frame color. It almost never works because the colors are close but not quite making the whole assembly entirely wrong. I'm thinking of something like a blue Homer with blue housing and blue tape, all in slightly different colors which add up to something that in conflict rather than harmonious.
Of course color, designs decisions and details are highly personal and subjective. There are instances of totally wild colors and combinations that I think work. Though I can't think of any monochromatic rigs that I have filed away as beautiful builds.
Do keep us updated with what you decided and share some pictures!