Yogurt is weird stuff. We buy the natural unflavoured organic (mostly for our B&B guests, and then we eat it if it is left over), and then if we want flavours we add jam or honey (which is the French approach)
We are currently experimenting as to when it is still good to eat after its sell by date. As we can't serve it to guests after that time we have a build up of yogurt for us to eat. I ate one the other day that was four months past its sell by date. Looked fine, smelled fine, tasted fine. :)
lol - you are your own science project! Plain is best far almost any food these days. Once they start adding flavors and colors - you are sunk!