What does "spark plugs appear yellow at the ignition point" mean?
Use the tip residue to evaluate the combustion process
The yellow the manual is describing is the normal condition found in the top left corner:

If the plug is not the yellow/tan shown in the top left, then it will almost certainly look like one of the other pictures. This is a strong hint that what is going on inside the cylinder is not optimal, and points in the direction of the problem.