The kanji used for Seiryu, 青龍 , can technically be translated as either Blue or Green Dragon, but the color kanji is generally read as Ao and taken to mean blue. It's not unheard of for Seiryu to be depicted as green, but most people in my experience tend towards blue, which Oda has clearly done by explicitly labeling Kaido as Seiryu and coloring him blue.
In the Four Symbols, the kirin is on occasion used in place of the Yellow Dragon, which again, I think is would be more fitting as Gold Roger in this scenario.
Still, the idea that Big Mom and Kaido are pulling double duty as the Dragon of the East, one as a true dragon and one as a kirin, is an interesting one, especially since they're the Yonko that are teaming up.
In the Four Symbols, the kirin is on occasion used in place of the Yellow Dragon, which again, I think is would be more fitting as Gold Roger in this scenario.
Still, the idea that Big Mom and Kaido are pulling double duty as the Dragon of the East, one as a true dragon and one as a kirin, is an interesting one, especially since they're the Yonko that are teaming up.