Don't know if it's anything like you're thinking of, but your post made me think immediately of this scarf:
http://ysolda.com/wordpress/skull-illusion-knit-scarf/
or this alein scarf which could easily be modified into a skull:
http://www.freshislefibers.com/Patte...ttinglinks.htm
I can't think how you'd do shadow or illusion knitting in a ribbing though. It pretty much depends on moving around the peaks and valleys of knit/purl stitches with color changes to get the effect.