it's just the color they happen to be. we're given literally no explanation for why they're there or where he got them from, but since he has them even in flashbacks from when he's like five, and because he always has to take them off before going all hypno-eyes on anything, i've always headcanoned that his dad gave him them to help control his powers when he was little, and he just always kept them because they were the only thing his dad ever gave him.