I use the Griseille method of painting quite a bit, both in oils and colored pencils. Griseille – a grey scale painting of your final image. In the case of colored pencils, I use it to control the intensity of a color by laying down a first layer of grey.
So, in Prismacolor, there are 3 greys. Cool Grey; Warm Grey and French Grey. Choosing Cool or Warm seems to be self explanatory, but when would French Greys be used? To answer that question I had to make a color chart with all Greys. It s shown below.
When all 3 are side by side, it seems to me that Warm Greys are more neutral, and French Grey is warmer. There are also strong value changes as the value gets darker in all 3.
I am experimenting with Lyra Oil based pencils in an effort to avoid wax bloom, so I added the grey range in those pencils also. There are 3 black. You can see that they relate to the Prismacolor range in one or the other.
I hope you find this interesting and will make your own charts as I do, in an effort to learn.