A blue-tinted gray door marks the entry to Billy's mindspace, but you'll have to think for a moment to set the cogs in motion that will open the lock. Luckily, there's a convenient string hanging to your left.

Within, if you choose to enter, the stylings are quite monochromatic. Three guesses which color it is, and the last two don't count.

Yes, everything inside this massive library is illuminated by an electric blue current. The light touches aisles upon aisles of books, a set of filing cabinets against the far wall, and scattered display cases of various inventions in Earth's history. You'll have to look high to find Billy, as he's currently standing atop a ladder in one of the aisles and retrieving an important tome.