The door blinks, its color changes in different colors starting from red followed by orange, then yellow turns to green and it became blue followed by indigo and lastly violet. The color changing repeats again.

They noticed that the door is unique, but there is no tag. This door is different from the former doors they have entered. There's no key nor tag to answer. Hannah memorized the sequence of the colors by saying it. Her friends heard her memorizing so, they memorized it ...