Subject: When two people see a colour, how do we know they perceive the same colour?

What I mean is, if my friend and I look at a red object, our brains 
interpret the colour as red. But he could be looking at a colour which my 
brain would have called green.
To take it further, how do we know that everybody in the world sees the 
same things as everybody else? After all, we cannot look into another 
person's brain and tell if what he sees is exactly what I see.
I hope I'm making sense here. Thanks.

Re: When two people see a colour, how do we know they perceive the same colour?