Titanium dioxide can be considered a "natural" colorant depending on your definition of what is natural. It's a mineral but needs to be processed to produce the pigment. IMO, I consider it "natural" since all colorants have to be processed in some way and TD is found in the earth. Other people may define "natural" as only colorants from plants.
BTW, there isn't a legal definition for "natural" colorants. Natural is a buzz word for label appeal. You see the word "natural" on a label and think since it's natural, it must be good for me. It helps to sell products. Organic is another word that is used for label appeal and the word "chemical" has developed a negative connotation. Everything is a "chemical"; although this is a simplification. Sodium hydroxide is a chemical and certainly cannot be considered "natural" since it has to be manufactured.
Please excuse my cynicism but I've seen too much misinformation, propaganda and abuse generated by special interest groups to believe what I read or to trust words like "natural' and "organic" as signifying something is a better product. However, I can understand the desire to eliminate possibly unnecessary ingredients from whatever you make.