is usually a convenience mixture of a black and one or two other colors that creates a cool dark grey. I don't find paints like that useful, because I can so easily mix them myself. On one or two occasions, I've encountered someone in an online art forum who avoids black because it is a "dead color," but uses Payne's grey. How is that different from having black on your palette?
Another good one is King's Blue, which usually a mixture of pthalo or ultramarine blue and white. What exactly is the point?
I try to pay attention to which pigments are in the paints I buy. I have a few multi-pigment paints left over from before I got more focused on color, and I still use those from time to time. But for the most part I stick with single-pigment paints. They provide more flexibility and a higher maximum chroma (because mixing paints reduces chroma). I also much prefer to work with and understand the individual character of particular pigments, which is something I can't do with convenience mixtures.