Aaagghh! I think they change the colours purely to increase sales so that people who have to have everything matching will buy more.

I started out with all ovals but found them really annoying for things like flour & sugar as my measuring cups don't fit in them easily and I'm to unco to pour out of them without making a huge mess. I know have quite a few squares which are sooo much easier for me to manage. I also have some mini rectangles which I love but they're not always available, sometimes they only come in big sets, sometimes you can't get them at all and sometimes they have them available to purchase individually as a special.