Absolutely! You can have the same design in multiple colors at no additional cost!
Simply, create the first design and add it to your Cart. In the Shopping Cart, you'll see a button to "Copy your Design". Click this button and you'll be redirected back to the Design Center. From there, you can change the design color (and make any other adjustments). When you're finished, click the "Add to Cart" button and both items will be added to your Cart. You can repeat the process for any other items.