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!).
ADDRESSES: You can choose to address your invitations by hand, or you can decide to print labels. Don't let the WIC bully you into thinking your own handwriting isn't good enough, or that the convenience of labels is tacky. As for the actual addresses, here are a few tips on how to address invitations formally, accounting for a whole host of last name conventions.