"Autosexing" is a term usually reserved for true breeding strains that are readily sexable. Welsummers are not autosexing in the same way that Legbars and Rhodebars are. I have Welsummers from Whitmore Farm (they are about 30 minutes from where I grew up and my Dad now lives) and there are a lot of chicks that are too ambiguous to sex with certainty. I make a good guess, but a percentage of the Welsummers "pullet" chicks I sell end up being roos. I tell my customers I am about as accurate as the hatcheries claim to be with vent sexing (90%), and that has been generally good enough. I also breed and sell Rhodebars and Legbars and the strains I have of both of those are MUCH less ambiguous, I can be 100% certain of those breeds. Now that I have recreated Welbars, I have a dark egg layer that is as accurately autosexing as my Rhodebars, and even better than the Legbars (some pullet chicks have small white dots on their heads).Your purpose is to create a new variety/color and to have auto-sexing identification -- although don't the Wellies already come auto-sexed as chicks per Whitemore Farms? I never could figure the reason to create an auto-sexing Welbar when the Wellies were already auto-sexed. But then there is something to be said for genetic hardiness in cross-breeding too.
This is a recent photo of the Welbar eggs I'm getting now. They fade a bit like Welsummers and Marans as the season progresses. They are identical to the pure Welsummer eggs.
Comparison of Welbars, Welsummers and Copper Marans (left to right)