"Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles."
So the theory would be, any duck with one copy of E and one copy of e+ would be neither, but rather some kind of in-between color. Like the blue dilution gene, which is incompletely dominant, making ducks blue with one copy and silver with two.
I can do a breeding next year to see if that's true

I have ducks I know are homozygous for E and e+. Crossing them should then have offspring which are Ee+, and then I can see what color they end up.