Lol, I just dropped by out of curiosity hehe. Interesting riddles there. Let's see...
R: Red
G: Green
Scenario 1
A: RR
B: RR
C: GG --> C would know right away
Scenario 1 is not possible or C would know right from the first round.
Scenario 2
A: RR
B: GG
C: RG or RR or GG --> But it can only be RG or else A would have guessed RR if C had GG or B would have guessed GG if C had RR
Scenario 2 is not possible or C would know right from the first round.
Scenario 3
A: RG
B: RG
C: RG or RR or GG --> But it can't be RR nor GG or else A would have guessed that, if B has RG and C has RR for example, he/she can only have RG and NOT RR (cause there would be 5 R then) and NOT GG either or B would see GG on A and C having RR, we go back to scenario 2 where B would be C and would have to have RG.
Considering the above scenario, C would ultimately guess that it has RG if B says nothing. Therefore, scenario 3 is not possible or C would know right from the first round.
Scenario 4
A: RR
B: RG
C: RG or GG --> If C had RG, B would know he/she either has RG or GG (thus B isn't sure). Same for A who isn't sure if he/she has RR or GG or RG. Since both A and B are unsure (and don't say anything), this means C, who also has two choices (RG or GG) is unsure as well. We learn that C does have RG then or we would fall under scenario 2 and B would have known.
Thus, in this round, C doesn't know from the first round what he/she has.
Now comes round 2. Since A has three choices, and neither B nor C answered in the first round, A can eliminate the fact that it has RG (or it'd be like scenario 3). A would know he/she doesn't have GG if C had GG and would guess he/she has RR. Since A doesn't say anything, this means C has RG. If that's the case, A would still hesitate between RR or GG and wouldn't say anything.
Now it's B's turn again. Since A said nothing and B can see that A has RR and C has RG from above, B can eliminate the possibility of having GG or C would have guessed from the first round that he/she had RG (ref scenario 2). The only possibility that remains is RG! B can then simply say "Red and Green"!
Note that it would also work if A had GG instead of RR. The same thinking process would have occured and B would have ultimately guessed that he/she has Red-Green stamps!