The term Titanium Oxide refers to the white pigment found in paint, and all other items that are white. It makes things white. A possibility is that the original paint you used had extra titanium added for better coverage. The Rust-Oleum Tremclan is a good anti rust paint and will work great for your door.
Make sure to clean and sand the door prior to painting. Sand a little more on the rusted areas for the best performance.