I agree with what beefcake said - and of course that "half way" could go in various directions, depending on the skin tones of the local human population.
Tolkien's half-orcs were "sallow", if that helps.
One point I'd make about the Mantic orcs is that they're not especially human in their anatomy - hunched shoulders, jutting heads and hugely thick arms and legs. So I'd be tempted to paint them in a variety of completely human skin tones and let the orcish anatomy do the rest. Giving them more human eyes might also help distinguish them.