I have spent a lot of time talking to Dave Bernard and others over the phone about a lot of things as it pertains to Cold Wax Medium and Gamblin Oil paints, as well as the things a lot of people like to add- like marble dust (calcium carbonate), cement, pastel powders, etc.
Artists Oil Colors and 1980 Oil Colors straight out of the tube are at their most stable. Adding Cold Wax Medium, marble dust or any dry pigment to a paint mixture is considered “subtractive” to its fat content which reduces the strength and flexibility of the paint film.
According to Gamblin Colors:
"A good starting point for considering how much additional dry pigment/powder can be added to a given oil color would be to consider how relatively fat or lean the color is straight from the tube. Gamblin Artists Oil Colors vary in oil content from 30-50%. While each color has its own unique formula, an easy indicator of relative fat content is to simply look at the color squeezed from the tube. If it is dull in appearance, the color is more than likely on the lean side. If it is glossy, the color is higher in fat content. Given this dull or glossy visual cue, bulk up the color with additional pigment by blending in a little at a time. If the mixture becomes too dry and pasty, add in a little medium to improve its consistency. I would recommend modifying the color with added pigment before adding CWM. Best to start with an oil color mixture that is stable before adding Cold Wax Medium."