I also use Ivory Black and Titanium White underpainting, and other versions, mainly Raw or Burnt Umber and Titanium White. I’ve looked around online and you can see people painting with blacks which are made up of reds mixed with greens, e.g. Alizarin Crimson and Viridan Green, or again blues and browns. At school I was taught by my militant impressionist teacher Mr. Baines that using black was sinful and one should only EVER mix blacks optically, and his chosen mix was Burnt Sienna and Cobalt Blue. You can mix a nice black from these two, but at the moment I think I can achieve a deeper black mixed from Raw/Burnt Umber and Ultramarine or Cobalt Blue. Having said all that, in the name of efficiency I have stripped back all of my processes to avoid unnecessary headaches so I generally I use Ivory Black and Titanium White! I have also learned from Louis Smith at the Manchester Academy that a little Alizarin Crimson mixed in with Ivory Black gives it more depth, so I also do this on occasion. The thing is that I can achieve this depth at a later stage by glazing darker colours over the underpainting so again the Alizarin Crimson gets ditched when I want to simplify the process. I’m possibly lazy, but if you are working on something over a few sessions and have to mix the colours each time inconsistencies can creep in and so thats just another layer of process that can be removed.