I tried a new scent out (Blueberry Vanilla from Daystar) and I kind of figured that it would brown due to the vanilla content, but I used some, not a lot, of titanium dioxide and blue mica for swirls in a test batch. You can see the swirls on top that went brown after three days. Even though the whole test batch will be brown as a bear, I captured the blue/yellow when I cut the loaf. It reminded me of those rocks that look plain on the outside and once it gets cracked open, there are gorgeous jewel-like stones inside. I've seen them in museums and specialty gift shops.
Sad to say, but completely expected... the soap is now fully brown, but it was fun to stare at it while it lasted. :)