As is the case with most vegetables, an easy way to gauge their antioxidant value - without knowing their ORAC value - is to base it off color. Not a steadfast rule, but often times those with darker and more intense colors test out with higher values. This pattern holds true with onions. When you look at a yellow onion vs. red onion, the latter has over 50% more antioxidants. However versus a white onion the difference is less profound; yellow is only 5% more.
ORAC Source
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010