What a hard riddle... It's like deducting
1st turn
1.
A=red stamp
B=red stamp
C=green stamp but didn't answer the first turn so NO
2.
A=green stamp but didn't answer the first turn so NO
B=red stamp
C=red stamp
3.
A=green stamp
B=red stamp but didn't answer the first turn so NO
C=green stamp
4.
A=green stamp
B=red stamp
C=*confused* he must have the green-red stamp
2nd turn
5.
A=*Confused* he must have the green-red stamp
B=reach the conclusion that he also have green-red stamp