Whereas there was a thing not long ago about rich folks buying basically painted cardboard for fake bookshelves! Or cultivated spreads of books they don't read in order to "look smart."
The bespectacled guy here was close to "getting" it.
He got it perfectly well. The books were for display and visual impressiveness only.
But unlike some people today, Gatsby probably didn’t color-code his books because he’s trying to look rich and well-read, not provide “a pop of color.”