Question on Question 20

I guess I am just a little confused on this one. In the game set-up, either G/P could be 1 because O cannot be 1. Same with 4, either O/P could be 4. P must be either 5 or 6. However, when considering if 1 and 4 being the same color would work, why wouldn't it? If we had G as 1, O as 2, P as 3, O as 4, G as 5, and P as 6, we are not violating any of the rules and still satisfying 1 and 4 being the same color. I am just wondering where I went wrong in approaching this game. Thanks!