[metacorrector]

the common color can be public, there is no point in manipulating it. Anybody manipulating it will not be using the common color any more.

if you are saying that you plan to trust the MITM to give you the common color, then it's not a common color, it's not something you have in common with the party you wish to perform a key exchange with.

or to put it another way, notice that the protocol does not start out with "exchange a common color"

[_ikke_]

If you check the cryptographic explanation, you see the first step is sending p and g to the other party. This is the 'common color' you talk about. These aren't predefined in the protocol.

[metacorrector]

my explanation was correct, "common" (but not secret) means that both parties have it (each party also has a secret) and that is the key element to understand what is good about the protocol and what problems it solves. By seeming to correct me, you are actually confusing the people I just explained it to. You can if you wish go ahead and explain what limitations this puts on the protocol, but try to help people understand rather than just throwing confetti in the air.