(in a nutshell, a gray card can lie).

A problem is that a gray card typically faces the camera, and gets frontal lighting. However, a face is 3d, and the left and right side can very much be getting different lighting.Another issue is when multiple people are in the same picture. I can recall situations with a group shot of 200+ people at a "Kids Camp" with some in sunshine, and others in the shade, and others under trees with "mottled shade". Nearly impossible.BTW: I have been using a SpyderCube from Datacolor, which I've found useful, especially since it was bundled with something else (pricey otherwise). I've asked Datacolor to consider improving it by having a "forward facing facet".Also, the SpyderCube is helpful with very contrasty Dual_ISO situations ... with one SpyderCube and a DIY black trap:I'll make it a point to look for difficult flesh tones.You may be aware of this, but my understanding is that, according to Dan Margulis (PS and L*a*b guru), Caucasian skin tends to follow a certain formula when viewed with CMYK ... don't recall the specifics ... Magenta and Yellow will be about the same, and Cyan about 1/3rd of M and Y? Or not?