You should be able to hit all the colors in that photo with the Zorn palette. First, establish the background accurately (why did you change it?). A straight ivory black/white mix should give a cool gray that’s very close to the photo, make sure you get the value right! For the complexion colors, I suggest you mix up several tints of black, red, and yellow with white and use those as the bases for your lighter mixtures, rather than bouncing back and forth between your pure colors. Identify the value of the area, determine whether it’s predominantly red/pink, yellow, or gray, and mix colors of similar value together. Color mixing is much easier if you approach it analytically rather than just blindly throwing colors into an ever-growing pile of paint!