A quote from (though almost certainly not Legends-canon) Star Wars Adventures in Colors and Shapes suggests that he cannot:
C-3PO: "Excuse me, Central Computer, but could you tell me if this plate is red?"
Central Computer: "No, that plate is green. Green is the color of grass, and of leaves on the trees, and of Greedo."
However, in Star Wars: Droids (also probably not Legends-canon), C-3PO solves a color puzzle.
In the Knights of the Old Republic RPG series, when you control a droid and view in first person they have monochrome vision, so perhaps that is the norm. However, I'd expect a protocol droid to need detailed vision.
I did not find much about C-3PO commenting on colors in a short search, and I don't remember anything from books. So, does C-3PO have color vision?