Hats in a dark room

[Susam Pal]

A puzzle from folklore: http://cotpi/p/15/

Arun, Bob and Charlie know that there are 3 red hats and 2 blue hats
in a dark room. Each enters the dark room, picks up the first hat he
gets his hands on and wears it.

After they come out of the dark room, they can't see their own hats.
They are curious whether they can figure out the colour of their own
hats only by looking at each other's.

Arun looks at Bob's and Charlie's hats and says that he can't
determine the colour of his hat. Bob looks at Arun's and Charlie's
hats and says that he is clueless as well. After Bob finishes saying
that, without even looking at Arun's and Bob's hats, Charlie exclaims,
"I know the colour of my hat!"

Which colour hat is Charlie wearing?