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Arrange a small cup or dish for each color of acrylic paint. Use an eyedropper or paintbrush to transfer a drop of paint to the liquid starch (or a small, separate test dish of starch left until still). Most brands and colors will sink, in which case you should add marbling gall to that container of paint. Vegetable oil is a cheap substitute, but may leave your paper a little greasy. Stir in one drop at a time, testing in between, until a drop of paint float on the surface and spreads out slowly. Stir in more if the drop rapidly spreads to more than 3 inches (7.5 cm).  Old-fashioned ox gall will not work with acrylic paints. Use a detergent-based marbling gall that does not contain animal products. This is also sold as surfactant or dispersant. Test each color separately, and test before each marbling session. Changes in temperature and humidity can change the amount of gall required.

Summary:
Mix your paint with gall or oil.