When looking at two ionic substances, we need to find out if there is a combination of the ions present that will result in the formation of an insoluble compound. In this case, both of our reactants, MgCl2 and sodium oxalate are soluble in water. The only possible products are magnesium oxalate and sodium chloride.
Since sodium chloride is soluble in water, the only possible precipitate is magnesium oxalate which has a low solubility in water (see link). A great source for finding the color and/or properties of a substances is to check out its MSDS (Material Safety Data sheet) which gives information about health hazards, chemical and physical properties, first aid measures for overexposure, etc.
White. the answer is white.