The Hooloovoo resemble a super-intelligent shade of the colour blue.

One was seen in a prism for Zaphod Beeblebrox's address upon stealing the Heart of Gold. On certain special occasions, such as the aforementioned Presidential event, they refract into a free-standing prism.

Their home world is a collection of asteroids in one of the far flung arms of the galaxy. They are able to "read" material like books by occupying prisms, including a special edition of The Hitchhiker's Guide to the Galaxy.

according to Hitchhikers Fandom Wiki

thanks to Douglas Adams for a lifetime of inspirations