Mixed white is simply a mixture of both titanium and zinc pigments, the result is similar to the creamy, smooth, opaque, consistent permalba white.
John Howard Sanden uses the Permalba White and so did Sargent, if you visit
http://portraitinstitute.com/promix.htm item "C" reads:
"Permalba White is preferred by artists for its delightful "buttery" consistency and "flowing" qualities. "
Some artists in Europe use the artichoke white, it because artichoke and permalba white are very consistent and never yellow with age.
Hope it helps.