Cyclops
Artistic depictions of the city of R'lyeh, home to the sleeping Cthulhu and perhaps other equally fearsome "Old Ones," are generally described as having Cyclopean features. This means that the proportions are large and somewhat humanoid, yet at the same time fundamentally alien. When Johansen arrives at the island he confirms the Cyclopean reference and compares the angry Cthulhu on the shore of the island to Polypheme, the cannibal Cyclops from The Odyssey.
The color green
Cthulhu is associated with the color green: his vaults in R'lyeh are covered in green slime (possibly due to having been under the ocean) and green ooze. The statue of Cthulu is made of greenish-black stone similar to soapstone, yet possessing golden flecks. Cthulhu himself emits a blinding green cloud.
Papers / manuscripts
Throughout the book, paper is associated with educated people and truth-seekers. It is among his great-uncle's papers that Thurston first discovers references to Cthulhu. Angell's papers integrate clippings from newspapers and other sources, and Johansen commits his story (conveniently written in English so that his wife would not read it) to writing on paper. This contrasts with the apparently illiterate Cthulhu cultists, who participate in oral chanting and worship a stone idol.
Dreams
Lovecraft uses dreams and dreamlike imagery to characterize the otherworldly forces at hand in the Cthulhu Mythos. Thurston first learns about the Cthulhu cult by reading about the dreams of a young artist named Henry Anthony Wilcox, who imagines ancient cities and a gigantic thing, "miles high." Angell discovers after recording the dreams of other poets and artists that these dreamlike images form a pattern.
