The yarn choice is pretty much a result of an archaeological-esque dig through my yarn stash. I'm probably one of few knitters out there with a dwindling collection of yarn that is in legitimate need of replenishing! I have plenty of bits and pieces, but not a lot of full skeins for larger projects, but that's why baby sweaters are so great! Rarely is a whole skein needed and you can easily split it up over body and edge to make the best of what you've got.
The darker color here is actually left over Koigu from my first sweater back in 2003! Koigu dyes their yarn in very small dyelots, so once you buy, chances are you'll never see that exact color again. I think it's a good idea to buy an extra skein and hang on to it. That way you'll have the right yarn should anything happen to your sweater, like a hole or buttons falling off. Actually, this is a good idea for all sweaters, not just Koigu ones! You could get really Martha about it and keep a little log book of all your projects with a bit of yarn and an extra buttons for each. I've done this when given a sweater as a gift, because buttons dropping off is kind of inevitable... if people are using their sweaters correctly!