update on these chicks. 3 of them are cocks (they sing all day long)
two are YH and two are RH,
now, those chicks who have a lighter-green back , one is YH and the other is RH (so the paler back colour has nothing to do with the head colour, nor does it have to do with being hen or cock.
These are the two paler-green backs, OH-PC on the left (cock) and RH on the right
the other OH, darker-green back (cock, also PC):
so, I still don't know the reason for those paler backs... could they be diluted??

?