I`ve been wondering about the same thing myself recently.. Some of my oldest roebucks have become a little bit yellow. I think the only way to prevent it on later mounts, is to do a really good job from the start.
I thought the yellow colour was from fat that had not been properly removed.
A taxidermist I know told me to gently wash the sculls with water, soap and a scrub. That will remove most of the fat, if you have done a good job cleaning up front. After this I let them dry for one night before I start bleaching.
In the bleaching process I cover the scull with cotton (on all sides), and pour peroxid over (38-40%).
After one night they`re as white as snow.
I don`t know if they`ll turn yellow later, because I`ve used this method only a couple of years.
Good luck!