Answer: yellow-orange are reserved days!

The yellow days are already reserved, but still in process. Therefore they can neither be bought nor seen, at least not in the moment.

These days might be user-pages, which we haven't checked and set on "free" yet; or these are user-pages, which still haven't been paid. It's also possible that this is a special date (New Year's Eve or similar), which will be sold (by auction) for a higher price.