Unpigmented "paint" is not white, it is usually* clear-ish. Titanium dioxide is the usual white pigment. On the topic of why iron oxide pigments are used rathe

Unpigmented "paint" is not white, it is usually* clear-ish. Titanium dioxide is the usual white pigment. On the topic of why iron oxide pigments are used rather than keeping paint clear: they protect the surface from UV light. * There are multitudes of paint bases, all the ones I can recollect are transparent with a yellow/brown tinge. One exception is acrylic emulsion paint base, which looks white but dries clear, unless a pigment is added.