When I made prosthetic pieces in theatre (noses, chins, foreheads, etc., more than full-alien masks), I went and bought my latex from the local ceramic shop. As near as I can tell, there isn't a big difference in the casting between ceramic and make-up/fx latex. The biggest is the base color, with make-up latex having a tendency to be flesh-toned. You can add a base tone to your latex using the same pigments you'd use for custom latex-based or acrylic paints. If you intend to paint the whole mask, this step is completely unnecessary.
Also, you can usually get larger quantities of ceramic latex for a lower price than you can make-up latex.
Caveat: while I'm allergic to many things, latex doesn't seem to be one of them. Neither "grade" of latex irritates my skin in any way to date. If you are concerned, definitely make a small patch and attach it somewhere before covering a large portion of your face with the stuff. Also, I never kept my pieces longer than a few months (the run of a show), so I don't know the longevity of ceramic vs make-up latex.