Here is one site I really like for colors: Design SeedsŪ | for all who
But it seems like finding color combos is not so much your concern, but how to use them. I never really gave much thought to this but I guess I do kind of have a method.
Usually when I'm using at least two colors, one is going to be darker than the other and I'll choose the darker one for the card base. The darker shade is "heavier" and if I used the darker color for matting, and the lighter color for a base it would kind of disappear, plus the darker color grounds everything. Then, I'll use a neutral (white, cream, etc.) for stamping the image/sentiments.
Of course, sometimes I'll pull colors from patterned paper. The base color may not match a color in the pattern but be a strong contrast, and the mat color would pick up on a more subtle color in the pattern.
For an example, if I wanted to use a deep blue, and a gold, and a cream color, I would use the deep blue as the base, and the gold for the mats. The pattern may have a lighter blue and yellow in it.
I do like strong contrasts and often rely on black, gray, brown to make other lighter colors 'pop.' You can also use neutrals as a narrow mat right behind the image/sentiment, then the coordinating color as a slightly wider mat behind that.
One thing that may be helpful is to visit the gallery here - find someone who uses color in a way you like and check out their uploads. Often people include links to their blogs here, too. Or go to your favorite stamp company's design team blogs or website or catalog ~ you'll find the cards appealing because you already like their products and it may be easier to relate to when you sit down to create your own. Discover a designer whose work appeals to you.
I often make my choices and then lay everything out to see how the colors look together - move things around and if it doesn't work, try a shade lighter or darker, or throw in a neutral in addition or instead of one that doesn't seem to be working.
HTH! Have fun experimenting!