If you want to have a color palette as a reference for your color grading, you can use the watermark module for that.
I’ve record a video showing the steps that you can do for that:
1- create a Palette of colors in Paletton.com
2- create a svg file with the colors of the palette in Inkscape
3- save the svg as watermark in Darktable folder.
4- import the svg file as a watermark. Those color appears in the vectorscope
Note: this can be used for a reference for the skin tones.