Eridan's Hive is formed from the wreckage of a large teal sailing ship sitting on a tiny island in the middle of the sea. The windows appear in a purple color from the outside, and the walls are teal rather than the standard grey. His hive being a sunken ship may be a joke on his romantic failures.
The building was later expanded by Feferi Peixes to reach the Seven Gates and eventually Skaia as required by Sburb, but was ultimately destroyed alongside Eridan's whole planet during Jack Noir's rampage.
Eridan's Respiteblock is not a closet under the stairs, just for your information. He has many wizard themed items in his room, including a fridge in which he stores wands. He has many maps for his plans to destroy the land dwellers, and lots of treasure from his pirate days.
All visible rooms in the ship with windows have a pink-purple glow (like the lightning outside) coming out of them, although his respiteblock is not lit with this color of light.