User blog comment:B214/Question and Answers 6/@comment-190.164.227.239-20161201033222/@comment-26821288-20161205080250

Now that I think about it, when Issei and Vali trained with Sun Wukong, I think he called Vali the Strongest White Dragon Emperor and he also called Issei the Strongest Red Dragon Emperor, so... Although I might be imagining things.