a snake texture mat--look in the modeling section of your local craft goods store
but you could easily do this with a knife--before you put it all together so you can hold the heel in your hand, get the coloring the way you want, patchy like that--then take a knife and gently score it both ways--take black powdered color and brush it on--then wipe it off to reveal the color showing through but it will stick in the grooves you made--
practice this a few times--you can do it
you could get this texture with netting too
Thanks! I will practice both methods. Thanks much.
So excited!!! I found it!! http://www.glasshousecakes.com.au/pd-silicone-texture-mat---snakeskin.cfm?OID=598