Welcome to the forum. I don’t know which one of Rose’s buttercream recipes you have been using, but I make her mousseline buttercream (pg 244 in TCB). I have made a few others, one of which was a chocolate version, so it wasn’t supposed to be white, but I’ve also made the classic buttercream and neoclassic buttercream and had the same issue with color. This is why I’ve made the mousseline as my “go to” buttercream now (plus it tastes absolutely wonderful!). I think the main reason is because the mousseline recipe uses egg whites, while the classic and neoclassic recipes use egg yolks (I could be wrong about this, though). Also, I use Darigold brand butter. There may be better brands available, but what I like about this brand is that it is a very pale yellow (compared to other brands available in my area), so it doesn’t provide a strong yellow color to the finished buttercream. I’ve found that when I make this buttercream it has a very, very pale ivory tint to it, but I have had no problems at all with coloring it with any type of dye/coloring agent. It’s also worth noting I omit the liqueur when I make this. Another nice thing about this particular buttercream it that it works great for piping. It’s soft, but yet still stiff enough for piping work if you plan on doing any. There are a few other buttercream recipes in TCB that use egg whites instead of egg yolks, but I haven’t made them, so I can’t comment on how white they turn out, how they taste, or how well they work for piping. There might be a recipe or two in RHC, but I still don’t have a copy of that yet, so maybe someone who does can recommend a recipe or two from that book for you to try. Good luck and I hope you find a buttercream you like.