Sorry I am unable to speak about what happened at that store. It may have just been miscommunication on the color or was sold before you got there. The offer for you to get it the next day is the same as if you purchased it from the website and had it sent directly to your house or for pickup at the store. They were probably just going to put that order in for you.
Instead of calling the store, you also have the option to go to the website to see if it is available at the store. You can purchase on the site and pick it up at the store. Currently it says that they are unavailable at the Apple Penn Square store, but can get it delivered by Thursday or of course pay extra to have it next day. The white one does show it is available.
Buy HomePod (2nd Generation) in Midnight - Apple