They are called "Tri Coloured" in the Australian and British Jap Standards. The colour/pattern is described as the following:
The colours white, black and brown or dark ochre should be as equally divided as possible on each feather.
Yet in the book 'Understanding Japanese Bantams" (John J. Palin, 1980), Plate 3 has a colour painting of a pair, and they have the same colour/pattern as Belgian Bearded Bantam Millefleurs (ie, not 'equally divided' between the colours), except the Japs have more white in the wing primaries/secondaries.
I don't think it matters much as to E locus for the Millefleurs, as there are some well patterned eWh
based Belgian Millefleurs in Australia, and eWh
based Belgian Millefleurs occur in the USA, and good Millefleurs occur in OEGB in the USA - some appear to be e+ based (not a surprise, considering Spangled OEGB are e+/e+ mo/mo based), eb
in other breeds/countries. The Aust. Belgian Standard doesn't exactly refer to feather undercolour, except the description of the Mille male neck feathers suggests a dark undercolour. The Jap Standard doesn't mention undercolour, so it wouldn't matter what E locus allele there.
The Aust. Belgian Millefleurs have at least eWh
and e+ based Millefleurs. I know Gerry Coady when redeveloping Belgian varieties, crossed Millefleur Belgian with Black-tailed Buff Jap. The B.T.Buff would have been eWh
based. If I was developing Millefleur Japs myself, I would use the best Millefleur colour/pattern Belgian I could get hold of (ie no excess white, good mottling, etc, but dark ground colour - ie stock with red enhancing mutation) and cross to B.T.Buff Japs. Make sure the Japs have no white at base of tail, in wing flights, etc. I wouldn't use Aust. Black Mottled Japs, unless they were very good mottled birds - the quality of excellent Belgians, Anconas or Spangled OEGB.