I have this headcanon that Rocky enters Grace classroom and sees that the pebbles spots are organize, but he can't tell how, so he tries to understand what was Grace logic behind placing the pebbles in that particular order.
He guessed size at first, but that is not consistent enough, he thought tone of "voice" but it is also mix, he even thought that maybe Grace was organizing them alphabeticaly by their human name, but it isn't that either.
When Rocky, defeated, gives up and asks, Grace admits that he organized them by color, he says it is too explain better how color works to his students, associating certain frequencies or textures with certain colors, but the true is that Grace just wanted to organize his classrroom as a rainbow.
