From the book: Valkyrie III. vs. Defender.
Official and signal program of the tenth contest for the emblem of international supremacy, the America's cup.
by E. G. Thorp.
From Harper's Weekly

VALKYRIE IN THE DRY DOCK - SHE WILL BE REPAINTED WHITE
When the Valkyrie arrived from England she was painted white, but while in the Erie basin (from 20 to 26 august) the white paint was scraped off and black lacquer substituted.
Tuesday September 3rd: The Valkyrie was towed up from the Horseshoe yesterday afternoon and docked in the Erie Basin. Lord Dunraven does not like the appearance of the yacht black, and she will be painted white before she is floated out of the Erie Basin. She will remain there until Friday, when she will be floated.

So Valkyrie III sailed in black between August 26 and September 1st.