The best lead tin yellow
Lead tin yellow is a very useful pigment for halftones. If you add white to a midtone, it can go too cold and the value will be chalky. If instead you add lead tin yellow, you can lighten without that problem. You could also use Naples Yellow but that will give you an opaque. If you want to lay down a semi-transparent veil, lead tin yellow is the pigment to use. The formulation sold by Natural Pigments is the real thing, which is rare, not some yellow mixed with zinc (which could lead to cracking). Natural Pigments sells a few versions, from a cold yellow to a hot yellow, and which you use depends on whether your lights are warm or cold. Being formulated from lead and not, say, a lake pigment, the paint is not fugitive. To me, it's a very useful, basic, and archival color that should be in every portrait painter's palette.