*** one TIP to remember first is that as long as the colors in the patterned papers coordinate, then it doesn't matter if you use dots, stripes, plaids, florals, solids etc. all on the same card... it will look splendid!
There are 2 easy ways to choose your colors....
1) Start with your already colored image for the front of your card, and then pick coordinating paper to go with it. (but this only works well if you have a bunch of paper on hand)
2) The best way is to pick your coordinating papers first, and then color your image to match.
Check out this video from Christina at Card-Making-Magic. She gives several examples that may help with the process:
Watch on YouTube: HERE
See that's not so hard, is it?