If you want to layer colors, fill the molds partially with one flavor and place them in the freezer for 1-2 hours, until the jello is set (like it would set in the refrigerator). Then, you can pour on the next layer and repeat the steps above until you have filled the mold. This process will take more time compared to filling it with a single flavor, but the pretty layered look and flavor combinations are worth the extra effort.