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.

Barnevelder

Eyy, that's near my home town! Barneveld (the town) is basically Chicken/Egg central, as we have companies that build the machines that wash and package our eggs. We also have Haantje Pik which is a sticky cinnamon-bun-like pastry. It's delicious!

You mean you don't treat them?