Answer to the Question Below

Answer to 'A, B and C':

The only one whose color can be determined is C.

If C's stamp were red, then B would have known that his stamp was not red by reasoning: "If my stamp were also red, then A, seeing two red stamps, would know that his stamp is not red. But A does not know that his stamp is not red because he did not see two red stamps. Therefore, my stamp cannot be red."

This proves that if C's stamp were red, then B would have known that his stamp was not red. But B did not know that his stamp was not red; therefore, C's stamp cannot be red.

The same argument, replacing the word red with yellow, shows that C's stamp cannot be yellow either. Therefore, C's stamp must be green.

If you don't understand this, you can ask me to explain directly...