Is Haki visible to other characters? We know of the several forms of Haki, the Kenbunshoku Haki, Busoshoku Haki and the Haoshoku Haki. I've been wondering, are those Haki visible to other characters in the One Piece universe, or only visible for the reader?

And I am mainly interested in the Busoshoku Haki, as this seems to harden the body and makes the skin look metal like. Can people in the One Piece universe also see this metal like change? Or is this just for the viewer to differentiate between normal fighting and Haki fighting?
Both Kenbunshoku and Haoshoku are not visible at all, since, well it was not made as something that is visually needed to the reader, since they're basically perception and sheer will power respectively.

Busoshoku, however, can be seen both by the user and others alike, though not all the time, since it acts like an "invisible armor" or coating, in a sense. At that point, it is not seen, which could be demonstrated by someone like Zoro, for example. But, when in is concentrated heavily on certain areas of the users body (i/e: Luffy or Vergo) or to a weapon of theirs (i/e: Vergo or Smoker), it hardens said part (or extension of the users body) and turns it black, making it "denser".    

(As for Vergo's extension for to his bamboo: Chapter 672, page 4 Smoker: Chapter 690, page 8.

For a use of Buso, without demonstrating it physically: Chapter 512, page 12, among many others. Zoro vs Monet for instance.)

Hope this helps.