It's very highly dependent on what brand/quality of the paint you use. If you use really high quality stuff like Vasari you won't need as much as if you use Utrecht or some other student-quality paint. Plus, student quality paints will surely have binders or fillers that may or may not be compatible with your varnish. You may want to consider foregoing the paint and mixing in earth pigments directly. Most oil paint is a pigment mixed with safflower oil anyway.

http://www.earthpigments.com/pigments-micas/?gclid=CPCZ0vfwhc0CFckehgodRbMO5A

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