What colour is a mix of two perfectly proportioned complementary colours?
As the answers in your other question (Why do complementary colours desaturate each other?) and the link in the comment by Danielillo address, it will in theory be a perfect grey: the complementary colours will have complementary temperature and value, so, after mixing, any pair of complementary colours will have the same perfectly neutral grey.

Of course, concerning the 'analogue' arts, in practice, other factors weigh in, like the quality and opacity of the pigment, and its coarseness. This will make the mixing harder, the proportions variable, and in most cases causes an overtone of the stronger colour.