I have no Ti-plants in my garden. That being said, giving the plants sulfur I believe would help you retain, return or develope the coloring. An acid soil reaction is what you are looking for. Adding some peat moss and animal manure composts as well.
I would also refrain from giving these plants any supplemental nitrogen. Let rain water be its source of nitrogen. I use a 7-4-5 fertilizer for acid loving plants on my crotons. Crotons color up better with a soil Ph of 6.8 or less. Nitrogen tends to really green up plants.
By the way, the next door neighbor has some Ti-plants. The last time they saw any fertilizer, Jimmy Carter was the President. They stay pretty damn red. Just my suggestion. Perhaps a few test plants to try my theorey?
Ron, No problem for me not to give them supplemental nitrogen. Up here around the bay fertilizers sold can not contain nitrogen.