Board Thread:Questions and Answers/@comment-25051789-20140627010334/@comment-24384892-20140629204900

His spiritual energy is blue, which causes his eyes to glow blue. It happens only when he emits a tremendous amount of spiritual energy.

It's the same when Kenpachi removes his eye patch. His eyes glow yellow, because his spiritual energy is yellow and plus, he releases an immense amount of it.