Georgia Pacific cardstock is sometimes too soft for colorists. If you have a excessive bleeding, give one of the other cardstocks that are recommended for alcohol markers a try. There are several on the market, Gina K makes a nice one, Crafter's Companion the maker of the Spectrum Noir markers has one that is awesome, Copic Xpress paper works great too. try one of these for less bleeding.
Jennie Eger Black
Alcohol Marker Specialist & Education instructor for Spectrum Noir