Thanks for the update Alan. This is really valuable info!
Here’s the colours you’re currently thinking about:
Rust-Oleum Colonial Red
Maybe you could borrow the pickguard idea from guitars, and have a protective section in the places most likely to have prolonged contact with the player? This could even be a fancy wood veneer.
I’d love to see a melodica finished in faux wood. Its a technique you can learn quite quickly apparently. You apply a base coat, and streak the grain on afterwards with a comb. Finish with strong varnish