For Bernard

John takes 8 stickers, 4 black and 4 white, shows them to his three friends Matthew, Mark, and Luke, and then sticks two to the forehead of each friend so that each of them can see all the other stickers except for the two in John's pocket and the two on his own head.

John then asks each of them if they know what color stickers are on their head:

Matthew says, "No."
Then Mark says, "No."
Finally Luke says, "No."

So John starts to go around again:

Matthew says, "No."
Mark says, "Yes."

What color are Mark's stickers?

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