After letting the underpainting dry, I now do what is essentially a completed value painting over top using a paint mixture of titanium white, raw umber, and mars black. At this point I'm still using little to no paint medium to facilitate blending and to keep with the fat-over-lean rule of painting (meaning that paint should contain more oil in the later layers than in the earlier ones to prevent drying discrepancies, and ultimately cracking). This step is probably the most time consuming because the goal is really to get all the values right. If you don't get it right here, it will get much harder to correct later on. One of the fun aspects to this step is seeing how much more dimension is created by the interplay of the cooler value painting over the warm underpainting.
Next Step: Glazing