I agree with Mystery that considering it’s a Faverolles mix that silver is probably responsible. Although, I suppose it’s hard to say if the silver doesn’t also conceal a dilution gene.
It probably wouldn’t surprise you to hear that these are mixed breeds, not brown Leghorns.
The one on the left is very spectacular.
I think he probably has mahogany and maybe the cream gene, though I’m not super familiar with what the diluters do.
Mahogany is a red extender gene like Columbian...