To activate any or all of the new color options you will need to prepare your artwork as PNG files with a transparent background. JPEGs are still accepted if you only want to offer a selection of lighter garment colors with no white ink underbase and keep a more vintage feel to the print.
Learn more about preparing your T-shirt files here: http://society6.com/help/tshirts
To upgrade your existing T-shirt posts to include our full range of colors, just go to that product page and click "ADD T-SHIRTS & MORE". Once on the additional products page, scroll down to the T-Shirts & Hoodies asset (Click "REMOVE THIS ARTWORK" if you have already uploaded a file here) and upload a new PNG file. Select your T-shirt and Hoody colors, check your product previews and you're all set. Due to additional labor and materials, the white underbase PNG option retails for a few bucks more than the JPG version.