While I don’t print so much anymore, I design invites from time to time and I offer letterpress/digital printing combos to brides who want to save a little bit of money. The color matching is a bit of a pain, especially if it’s running on digital and not offset where you can specify the Pantone color to match. I recently did a job where the bride wanted letterpress invites and digital printed RSVP/info cards, I had the digital printer print me a special swatch sample page on the digital Lettra I intended to print on and I found a close match to the letterpress prints, only to get a call from the printer that said “um, these are printing grayish-purple” — I was trying to match a coppery/gold color. My bride was flexible and we were able to switch to the closest paper we could get to the Lettra.
My advice is…find a few local printers you can trust, make sure you let them know you’re trying to match to a letterpress print and provide them with a sample of what you’re trying to match (that’s what saved me!) and CYA by mentioning that the color match may not be exact, like Val said above.
Good luck! 🙂