milk (aka dark envelopes) was a bad choice

Maybe you can use an embosser?

Oh wait, that probably won't be visible enough. Sorry!

You could use a clear address label with white ink

Is handwriting absolutely out of the question for you? You could do silver or light colored marker or pen. Maybe yellow?

We just did my besties dark envies with silver sharpie (autograph pen) and it look awesome but we also hand wrote all of hers!! but we also toyed with the idea of have white block card stock on front and glued because we were having the same prob as you

I vote handwritten like the girls above.

I think handwriting is out of the question due to the # of guests I have (which is a lot). I asked my calligrapher to do the handwriting and she even said no because there are so many!!

I agree with Robin - JoAnn's or other craft stores have nice markers/pens in the scrap-booking section that come in silver, gold, bronze, and other metallic shades. I used these to address all my envelopes and it looked really nice.

ETA: Just saw your post. Is there a way someone could create a stencil so it would be easy for you and some friends to do it? I don't even know how well that would work, but maybe it's out there? Otherwise, perhaps an address label and hide the edges somewhat with rubber stamps?

Dark envelopes here. I'm handwriting using jelliroll pens in milkyteal. Shows up pperfectly! Return address will be a stamp in the same color.

I didn't realize there was etiquette for using labels. I guess I violated. I used the clear labels, but while on white they will appear clear, on darker colors you can see the filminess of the label. Not too bad tho, IMO.

A stencil is an idea I had not thought of ... thank you!!

There's no such thing as white ink, well for an at home printer that is...

I had dark purple envelopes and I bought a return address stamp off of etsy for $20 and used a white ink pad. It worked great, was super fast and looked awesome. Gold and silver ink pads also worked, I tested all of them, I'm using a gold ink pad for the invites.

So good to know of your past experience. Thank you Katie C.!! Do you mind sharing with me the name of the seller you used on Etsy? There are sooo many who sell custom rubber stamps, I'd really appreciate a personal recommendation.

A colored gel pen (sliver or gold)

I had burgundy envelopes and we used wrap labels. I don't care that etiquette dictates things to be handwritten. I think wrap labels are nicer than regular. I am on my phone, so I don't have a picture, but you can do a google image search.

A stamp with a metallic ink would work.

Have you tried printing it at home with black ink? Maybe then you can trace over it with a gel pen

I used clear labels with a really nice calligraphy script for both my invites envelopes and RSVP envelopes. It's totally fine..

I would use clear labels with maybe a gold ink or bright colored ink that would show over the brown

You could create your stamp on vistaprint, but don't use the ink pad inside of it that allows it to "self-ink"..

Then make your own clear embossing ink:

http://suzieridler.blogspot.com/2012/02/cheap-easy-homemade-clear-embossing-ink.html

Use white embossing powder, apply the heat gun and voila!