There are numerous genes at work with Welsh Harlequins. A lot of the original research done on duck colors was all the way back in the early 1900's and some of the original studies were on only a few pairs of birds so there is probably a lot we don't know.
Basically, Welsh Harlequins carry dusky mallard, light phase, and harlequin. The Gold Welsh Harlequins also carry sex-linked brown dilution. Dusky and light phase are seemingly not completely recessive which could result I think in several variations in phenotype (not to mention all the unknown genes that may be at work).
If light phase and dusky are not completely recessive, I don't think there is really a "name" for heterozygous dusky phenotype or light phase. It becomes very complicated and unfortunately, unless a person completely recreates the breed in order to absolutely know the genetic make-up of their birds- it will probably always be as clear as mud. I have nine WH that are still feathering and it appears, at least initially, that they are going to have at least three distinctly different plumage colors/patterns.
Other colors in ducks, like Blue, do have names for the heterozygous verses homozygous dilution.