It should open by a password or talisman, but you don't know the answer to the riddle even though you spent weeks one time scouring the story of the Orchard and its commentary for encoded messages. You assume the gray is supposed to represent The Other One, whose loss in the Orchard was the worst: they kept their life and their mind, but when they destroyed the saplings they lost their name. Why your creator felt this was worth memorializing on the door to the room you always assumed they slept in, you can't fathom. Only now you kind of have to.
The thought comes into your head, mysteriously as if from nowhere, that the password may be the encoded form of the color that The Other One lost.