I am new to wild type down and barring but why would being double barred lighten the color of the chicks??? Barred rocks and cuckoos even golden cuckoos are not lighter at hatch with double barring. The lighter look comes later as the barring starts to express. The yellow is WHEATEN the dar ker chicks didn't get 2 copies of recessive wheaten.
It is due to the E locus. in barred rocks it is the Black allele E/E for the down color and even though it the dosing effect doesn't lighten the down except for making the head spot bigger, it does inhibit the black pigment in the shanks causing males to have lighter colored shanks than females in barred rocks. But on the wild type and some others of the e locus the whole chick is affected by the dosage effect.
I will disagree with your about the leg color on BR..... my Heritage Barred Rocks don't follow that at ALL..... neither do they follow the bigger head spot. I can't sex my GSBR at all.... never have been able too. Hatchery BR birds yes. Cuckoo Marans I can sex mostly by the spot.
Can I ask why we keep going back to RIR? Why not Barred Rocks???? Could you put your Rhodebar Roos Barred Rocks? If they are single barred and need type and size could you not work that direction?
The rhodebar was developed (in theory) from Brussbars and RIR. They should be an autosexed, barred RIR.
The RIR type is what you are looking for. Barred Rocks have a very different type. Could you create an auto sexed bird with infusing Rocks? Yes. But in my opinion it would not be arhodebar.
Remember the "reason" some of us are infusing new RIR blood into existing Rhodebars. .. Because the Rhodebars need to blood to improve and RIR "could" provide that improvement if chosen carefully.
There are a lot of crosses you could do to create a new auto sexed breed. .. we are not trying to create a new breed, we are simply trying to improve am existing one.