My understanding of blacks and yellow are (i have no run them or held them so this is from a few different jaco guys)...
black- its the shore rating of a purple however on jaco tires they do not pick up traction as much through the run. I have tried parma blacks at big races and found that their purples dont pick up traction as much as jaco purples do
yellow- a shore rating between a black and grey used in the rear.