I don't know if you're still looking for answers on this, but as far as sealing copper, I've had success with spray lacquers. I mostly use a Krylon gloss lacquer or one that's labeled as for brass instruments (I work out of a studio at my local college a fair bit; the instrument one is one of theirs, so I don't know the name). I'm fairly new to mail, so I don't know a lot about the durability for that application, but it seems to hold up pretty well on bowls I've made, including against some contact with sandpaper.
If you use smooth-face pliers, it should work to seal the individual copper rings before you actually make the mail. Once it's assembled, it can be hard to get any cleaning or sealing agent into all the little bits, but it can probably still be done. For that, I'd suggest doing multiple coats and trying to move the rings between coats, though that's just a guess (I typically seal my work before I do cold connections or expect things to stay unsealed).
As far as coloration:
Bright pink/red: You can clean your copper in a vinegar/salt solution or a pickling solution (mostly used for hot work), rinse it in a baking soda solution, and spray it immediately after you dry it. I haven't tried it with vinegar/salt, but a used pickling solution also works really easily to copper plate other metals (simply put your piece and some steel in the used solution). If you copper plate at home, it will come out to this color, but you should be able to color it to any of the colors below. The plate will be pretty thin, but in my experience holds up okay to light cleaning. Brass and bronze can also be copper plated by heating them with a torch until they're bright red. That will anneal them to dead soft, but dead soft for bronze is still pretty hard in my experience.
Bright orange: You can get a bright orange out of copper by putting it in a toaster or oven at 275F for about 20 minutes. It does seem to be fading slightly with age, but the seal on that has stood up well to wear as far as I can tell.
Antique brown copper: You can get variations on antique copper in a couple of ways. The most durable way is probably to just let the copper age until it gets to the color you want and seal it there. I can get a pretty good brown from that freshly-cleaned copper within a couple of weeks, but I'm in a warmer and probably more humid environment than you. Peanut oil can be used in a heat patina that turns out as an antique copper, but I haven't done much with it to know how durable the finish is. Rub peanut oil on the piece and heat it until the oil starts to smoke. This method doesn't really seem ideal for mail to me: a ton of work for individual rings, and a really inconsistent coating for a completed piece.
Iridescent to black: You can get an iridescent to black with liver of sulfur. A really light touch on liver of sulfur can also get an antique copper, but the color changes pretty quickly in my experience. It holds up okay, but it works best on deeper areas that aren't going to get a lot of wear, so I wouldn't recommend it for mail. It's especially brittle if you let it go all the way to black. I would definitely caution against that. Typically, the color will proceed brown to purplish to iridescent to dull black.
Green: I haven't tried antique green. I'm told muriatic acid works to create that color, but I have no personal experience. Very likely going to be brittle, though, and I'm also told it can be toxic.
Bright blue: This one is fairly brittle, but it's really dramatic. It creates an uneven almost royal-blue coat. Wet the piece and cover it in salt (different grains will have different effects), then suspend it in a closed space with a little cup of ammonia. It takes 1-2 days to color the metal.
Again, I'm new to mail, but I've been making copper jewelry for a bit. Most of my experience is with more solid pieces that won't have as many moving parts or much inherent rubbing together as mail does, and while I've tried to guess how the colors will wear with mail, it's all extrapolation.