I would choose an orange fabric for the border. You could use a print that's swirly like the purple or mottled like the green. Or something different from both. I'd choose orange because 1) it's unexpected, and 2) it's the other "secondary" color on the color wheel. (Primary colors are red, yellow, and blue. The secondary colors are derived like this: red + blue = purple; yellow + blue = green; red + yellow = orange.) I guess I should add 3) I love color! The more the merrier in my book!