Misa's eyes brown or blue?
Every other time you saw her her eyes were either brown or blue.
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Maybe both. The eye colour "hazel" can change drastically based on clothing and environment. Mine can look blueish, green, or grey depending on something like weather. Sometimes I get people guessing brown, too, but not often. So, if Misa has hazel eyes, they might shift.
Either that, or it's a side-effect of the Shinigami eyes.