Co is incompletely dominant with e+ and eb hens. There is remnant stippling on the back of both hens.
Co/co+ expresses more with males.
Then there is Db that gives similar phenotype in hens.
Don't know about Di - might do similar?
Then the quail pattern (eumelanised and with eumelanin restrictor), can produce similar phenotypes (hets. - eg quail cross columbian -tends to be incompletely dominant in adult phenotypes).
Plus, silkie feathers with dark undercolour - tends to show through more to surface.