Titanium dioxide (TiO2) presents one of the more difficult color-removal challenges, especially when present at high loadings. When faced with the need to remove a white material carrying a high level of this pigment, an effective pre-flush is very important.
Use a clean, natural material that is stiffer (i.e., higher in viscosity) than the pigmented production material (regrind is fine) and then flush until – in your judgment – the bulk of the white color has been removed. Then finish up the job with a good purging compound. And remember to run the screw empty between materials so that mixing dilution doesn't hamper effectiveness.