This color combination works so well because they are complementary colors on the color wheel:
My neighbor Patti has used this combination beautifully in her rooftop garden:
Classic plants for this that both Patti and I have used are the bright green sweet potato vine and a red-violet petunia. Both types of plants are super easy to grow. In place of the sweet potato vine you could also use a lime green coleus or one of my favorites, the licorice plant. For the purple flowers you could use a calibrachoa.
So if you don't have any flowers in your outdoor space yet, get out and buy a couple of these inexpensive plants for a no-fail beautiful pot!