Some purpling on the lower stem and on the undersides of leaves on new plants is perfectly normal and okay. Deeper purple stems up the entire stem or purple veining leaves in older transplants can be an issue. A well balanced NPK liquid fertilizer at 1/2 strength and some Epsom Salts will fix your plant and restore the green.
Check out my vegetable gardening blog: The Rusted Garden. It is filled with garden information, videos, pictures, seed catalogs and seeds & things I sell. www.therustedgarden.blogspot.com
Join My Google+ Gardening Community called Our Tomato & Vegetable Gardens – we are approaching 5000 world-wide gardeners: or Link from My YouTube Page.