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Used to be, since green is (apparently) the color of willpower and yellow is (apparently) the color of fear, so in order to use his ring on yellow stuff he had to conquer his fear or something. Eventually that got explained away because he got so good at using his ring that he's not affected by yellow anymore. Or something. I think they just realized what a terrible weakness it is.
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Apparently they pulled a "It's all in his head, his weakness was never real" cop out.