Present day thinking about colored heads on dogs who have the piebald gene, is that there may be a gene or gene modifier that is linked with piebald gene that causes colored heads:
"It is not clear to what extent the S series affects head pigment. Color-headed white shelties, for instance (swsw), can have completely colored heads - not even a forehead star or white nose. On the other hand, relatively conservatively marked dogs can appear with half white or all white heads. There is probably at least one other gene series that affects head markings. It is at least possible that the plus and minus modifiers affect head and body markings simultaneously."
Coat Color Genetics
As for writing a standard for parti coloring, I still believe that will be a hard one to do if you get too specific due to the random nature of how the spotting works.
The Toy Fox terrier has a pretty detailed standard:
Color
Tri-Color: Predominately black head with sharply defined tan markings on cheeks, lips and eye dots. Body is over fifty-per-cent white, with or without black body spots. White, Chocolate and Tan: Predominately chocolate head with sharply defined tan markings on cheeks, lips and eye dots. Body is over fifty-percent white, with or without chocolate body spots. White and Tan: Predominately tan head. Body is over fifty-percent white with or without tan body spots. White and Black: Predominately black head. Body is over fifty percent white with or without black body spots. Color should be rich and clear. Blazes are acceptable, but may not touch the eyes or ears. Clear white is preferred, but a small amount of ticking is not to be penalized. Body spots on black headed tri-colors must be black; body spots on chocolate headed tri-colors must be chocolate; both with or without a slight fringe of tan alongside any body spots near the chest and under the tail as seen in normal bi-color patterning. Faults: Color, other than ticking, that extends below the elbow or the hock. Disqualifications: A blaze extending into the eyes or ears. Any color combination not stated above. Any dog whose head is more than fifty-percent white. Any dog whose body is not more than fifty-percent white. Any dog whose head and body spots are of different colors.
Say the parti's would come up with a standard like this, what happens if a pup is born with some of the disqualification markings? Is that pup not registered and should be to be spayed and given away as a pet, or is that pup just not suitable for showing?