A Set of Stamps

A moderator takes a set of 8 stamps, 4 red and 4 green, known to three logicians, and loosely affixes two to the forehead of each logician so that each logician can see all the other stamps except those two in the moderator's pocket and the two on his or her own head. He asks them in turn if they know the colors of their own stamps:

1. A: "No"
2. B: "No"
3. C: "No"
4. A: "No"
5. B: "Yes"

What are the colors of B's stamps?

Source: Internet newsgroup rec.puzzles
[Taken from Ken's Puzzle of the Day, March 16, 1994.]

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