The advantage of underpainting is that you can quickly get the features in the right place and produce the right structure and tonal balance. It is monochrome so you do not need to be concerned with colour at this stage.
I use Terra-Verta for my underpainting of flesh which is easy to use and cools down the finished painting where required if showing through.
Once the outline sketch with pencil is on the canvas I mix my three tones of Terra-Verta. One staight from the tube, one with a little ivory black and a third with a little more ivory black. I never mix white into the mix. Using these three tones, turps and a watercolour brush I start to apply the Terra-Verta very diluted with the turps. I paint all the flesh areas by gradually building up the layers of paint with less turps as the layers increase to obtain the details, structure and tone. If when I stand back I see I need to change something it can easily be done by moving the paint around with turps. I continue until I’m happy.
I put away the Tera-Verta and turps to begin the next stages. The Terra-Verta and turps do not come near my pallet again.