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When you tap the Summon button on the summoning screen, a particular Hero (and their corresponding color) is chosen for each summoning stone according to the appearance rates for each rarity (totaling 100%). Of the five summonable Heroes chosen, the color of the summoning stones is not determined first. Rather, the colors of the five summoning stones that appear correspond to the colors of the five Heroes who were chosen when you tapped the Summon button.
Depending on the value pulled in the beginning of the process, a summoning stone of each color will not always appear, or you may notice more summoning stones of certain colors appearing more often.