Use a neutral colour for the cards themselves, then add colour through printing, embellishments or ribbons, or use coloured card and make a great statement. Card textures can make a big difference to the overall feel of an invitation, from rustic matte textures to contemporary pearlescent effects to vintage tapestry styles. Consider printing or foil stamping the front of the cards, or use peel_off stickers (with a steady hand!).
However, if a paper cutter sounds like a useful thing that has a place in your office, buy it! You don't need to shell out 贄 for a good one. This ษ trimmer has been a great little workhorse for me for about four years now, through invitations, photo albums, and many, many other late night print jobs.