Watercolor Dirtbag
Neutral Tint is a transparent gray (black in masstone) that is specifically designed to be neutral: not warm, not cool, not leaning toward any other color.
Typically, Neutral Tint is made from a mix of three pigments: PBk6 (Lamp Black), PV19 (any number of quinacridone magenta/pink/rose/purple/crimson shades), and PB15 (Phthalo Blue of some sort).
Shadow Violet is a granulating purple-gray that’s made from a mix of three pigments: PB29 (Ultramarine), PG18 (Viridian), and PO73 (Pyrrol Orange). As such, close inspection of the seemingly unassuming shade reveals flecks of violet blue and blue-green as well as and underlying orange cast that make it more interesting than your typical gray, and mimics the overall effect of a real-life shadow with light and dark spots, color variety, and texture. John Muir Laws praises its beautiful granulation and suggests using it as a convenience gray for shadows in nature paintings.
Payne’s Gray is a blue-toned dark gray (or, in some brands, a navy blue). Different companies have different formulations, but the granulating Daniel Smith version pictured above is made from a mix of Ivory Black (PBk6) and Ultramarine Blue (PB29).