What do the room colours mean on the front desk board?

• Green — available, ready to sell.
• Blue — occupied.
• Yellow — due to check out today.
• Red — overdue: past the checkout date and still accruing nights.
• Grey — checked out, waiting to be cleaned.

Red is information, not a fault

An overdue room means the guest has not been checked out. Either check them out or extend the stay, so the bill matches what is actually happening in the room.

Grey rooms cannot be sold

That is deliberate — a room is not offered again until somebody has been in it. Housekeeping clears it from their own screen, or the front desk can mark it cleaned from the room's menu.