This is an interesting topic. I met with a merino children's clothing manufacturer a couple of months ago and this very issue came up with them. They mentioned that they get a lot of people trying to return garments because they have yellowed, and that they find it very hard to explain to people that there is really nothing that can be done, because merino is naturally going to yellow over time (as it's worn and gets softer) because it's a natural fibre and there is no way to "whiten" it without either adding another fibre to it (a merino blend rather than pure merino) and chemically treating that fibre, or putting it through some kind of scary chemical process to try to artificially brighten the cloth (which you would not want next to your baby's skin afterward, apparently). So in essence, there should never be any pure white merino clothing, because it's not natural.

I thought this was fascinating, because we have looked at a lot of merino clothing for fashion shoots for the magazine, and we have never seen any that was white-white. So the reality may be that you just have to be stuck with yellowed clothing, because trying to whiten it will destroy the delicate fibres. I've also read that detergents with optical whiteners should not be used on merino either, as that degrades the fibres faster and causes them to weaken and fray.