The most simple techniques can often yield the best results. Here is a simple watercolor technique that will help you create the most elegant card. Naturally, you should use watercolor paper for this technique. I also recommend that you use a heat tool. The paper may curl a bit, but when you dry it, it should flatten out.
You could use different colors for different themes. Blue for boys and masculine cards, pink for girls, greens and yellows for Easter and summer themes, oranges and browns for fall.
Have you ever made a watercolor ombre card?