However, Paboy, when you book that "purple" hotel, the website gives you the price for the night when you put in the dates, and holds that price until you complete the booking. the price doesn't change during the booking process, which is what the OIP seems to be saying.
And although in English contract law, there is the general principle of "invitation to treat", there is additionally specific consumer legislation regarding advertised prices in retail operations which over-ride this. If you go into your local supermarket and pick up a tin of beans priced at 10p, but the electronic check-out charges you 12p, the supermarket is in breach.