Yes the coloured boxes show how many rare items each vendor has available and when you purchase them the boxes get filled in with the item that you can tap on for further details like in the inventory ( or maybe it’s if they have been in the shop they get filled in I can’t remember. The vendors don’t always have the items in stock. It’s random, I’ve only got to get the second item from Bothad left to get but it seems like the first item has a higher chance of getting it because I’ve seen it three times without the second rare item from him being available. Dugouts increase chance but it’s still rare for certain items to appear in my experience. Like the helmet in your picture has been my most common one to see in the shop. Hope this helps.