The colors of Christmas are red and green. These being fairly bright colors, you would want something more subtle, like white or silver, to be the theme of your Christmas wedding. That way, your decoration complements the colors of the season. You should probably use the same color as your theme on the invitation card. Just because it's a Christmas wedding, don't fall into the temptation of using a holiday type card as your invitation card. Again, use something subtle and elegant for the card. An ivory card with gold trims would great. The gown for the bride will probably be the same that would have been used anyway.
The idea is to have her look her flattering best. You could add a tinge of Christmas color with a red cape or a red silk sash around the waist- just enough color to remind the season, without overshadowing the wedding. For the cake, you have many choices. You could still use a traditional tiered wedding cake. However, for your Christmas wedding, you could make it a bit special. You could make it look like a tower of wrapped gift packs with a filling that's traditional for the Christmas season – maybe marzipan, which is an almond paste