This excerpt is taken directly from the Belgian Bantam Club. So really, "gold necked" is a misnomer. The same thing occurs in breeding silkies. The homozygous form of blue is called a "splash." THAT'S why I called my gold-necked a "Splash".
It is my understanding that to be admitted into the Standard of Perfection as a recognized variety, said variety must "have a relatively large sample size." To get to that sample size for the current accepted varieties, breeders used those that bred "true". Meaning that 2 birds of the same color, crossed, result in the same color. Goldneck is a recognized variety of d'Uccle because 2 goldnecks consistently create more goldnecks (in my experience this was proven true), and so breeders all over the US could show a consistent quality of bird in all categories (K,P,C,H) in each sanctioned show for admittance into the APA Standard.
Blue mille is not recognized because they do not breed true. Blue & blue mottled are also not recognized colors, or varieties, in the d'Uccle section (or booted bantam section) of the APA Standard of Perfection. Splash is also nowhere to be found in that section of the APA Standard of Perfection. All can be shown, but only as Alternate of Variety, and they cannot make best or reserve of breed.
So, I hold true to my original statement, while respecting your right to disagree.
I wish I had more time to go back to my poultry genetics book and relocate some cool quotes about d'Uccles and their genetic 'soup'. I think the history behind d'Uccles and their breeding is fascinating!
Sadly, I have other research that is required, for a grade.....
Edited: Because I found my notes from a long ago talk with a breeder who helped get some varieties admitted to the standard in 1996. It was during a discussion on what it would take to get Silver Mille recognized & admitted.