From what i understand just a diet more rich in beta carotene will produce a richer looking yolk. Seems like the chicken’s lifestyle would have other effects, too. And yeah, in the US eggs come throughly washed, which removes a layer on the outside that would otherwise keep them fresh at room temp. I think the salmonella thing is more related to the sanitary conditions of the farm - I.e. whether the chickens are infected with salmonella. Farms have cleaned up in that respect over the past couple decades and it’s much less prevalent than it was at one time.