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!).
Start with the basic invitation _ There are loads of different styles and types of cards, depending on your budget and how much information you need to include. When you order your samples, try to make sure you have some of each style to see what would work practically for you.