NFHS Football & Basketball
Dread Our Coming, Suffer Our Presence, Embrace Our Glory (Solonavi War Cry)
I always thought they were referred to the "Black Fleet", although that might be more metaphorical and not literal.
From the Shattered Empire rulebook:
"Salai could see the emerging outlines of the Sol fleet. Like a vast swarm of black birds, the vessels soon shrouded the stars."
Well, based on that quote, it sounds like it was dark out, so they would be dark shapes, and even if it was day, seeing the ships against a bright sky at a distance would likely make them look black.
I don't think there's much in the lore in either TI3 or Rex to specify the exact color of any of the ships other than metaphorically, although in TI3, those who paint ships for each race tend to make the Sol ships a light blue with yellow highlights.