Most of the posters have had good information. One guy suggested that the yellowing may be caused by grease leaching back out of the bone. This is correct. The skull needs to be completely de-greased prior to bleaching, or it will continue to yellow.
Use Sal soda, available from Vandykes Taxidermy supply or others. It is sometimes called washing soda. Boil the skull in water and Sal Soda to degrease it completely. Be careful to not over boil the skull, the smaller bones and teeth will come apart and fall out of the skull if over boiled.
After degreasing, make a paste from 40 to 50 volume peroxide, available from the beauty supply store, and magnesium carbonate, also available from taxidermy supply businesses. I believe that the kits sold at Sportsman's Warehouse contain these two items.
Wear gloves for this step, the peroxide and chemicals will burn your skin. Put the paste on the skull with a cheap brush, bag the skull in a garbage bag, and place in the garage for a day. After waiting a day or two, thoroughly rinse the skull and place in the direct sunlight for several days. This will result in a nice white skull with professional results.