From The Fellowship of the Ring (book), this passage at least mentions the color.
As for Bilbo Baggins, even while he was making his speech, he had been fingering the golden ring in his pocket: his magic ring that he had kept secret for so many years.
As does this from the prologue, describing Gollum:
He possessed a secret treasure that had come to him long ages ago, when he still lived in the light: a ring of gold that made its wearer invisible.
Also from Wikipedia:
The words of the ring-inscription are in Black Speech, a language devised by Sauron and used in the land of Mordor. The inscription reflects the One Ring's power to control the other Rings of Power. The writing uses Elvish letters (tengwar), in a mode adapted to the Black Speech.
The Ring had the ability to change size. As well as adapting to fingers of varying size, from Sauron's to Frodo's, it sometimes suddenly expanded in order to give its wearer the slip
The size and shape is not as concrete as its color.
Normally the One Ring appeared perfectly plain and featureless, but when heated its inscription appeared in fiery letters. A drawing of the inscription and a translation provided by Gandalf appears in Book I, Chapter 2 of The Fellowship of the Ring, "The Shadow of the Past".
Gandalf speaks the words in Black Speech in Book II, Chapter 2, "The Council of Elrond":
"Ash nazg durbatulûk, ash nazg gimbatul,
Ash nazg thrakatulûk agh burzum-ishi krimpatul."
Translated, the words mean:
One ring to rule them all, one ring to find them,
One ring to bring them all and in the darkness bind them.