I asked some questions about titanium dioxide the other day. What I was told was not to use more than 1 tsp. PPO or else it makes the soap chalky!
I find that 1 tsp. PPO isn't really enough for some really dark discolouring FO's though. I used an FO that turned my soap bright yellow tonight, and for a 3.5 lb batch, I used 4 tsp. of titanium dioxide and the soap still didn't turn out white. It is way lighter, but not white!
You can also use white kaolin clay in your recipe to lighten it a bit. It probably won't make a huge difference but it'll help a little. I've done it.