Responses
I think he loves Riya without seeing him as black or white, but he respect the white riya who loves his senpai (at least he knows it like that) because he loves riya no matter which one he is. He doesn't want to push the white riya if riya doesn't want to. He think like the white riya maybe can't accept this and can be upset and he doesn't want that happen. That's why he upset, not because he doesn't loves the white riya when the black riya disappeared but he upset because the white riya that he knows don't love him even though he loves him too. Well this just my opinion
But He obviously fell in love with black. The cat he was waiting for is black. He gave the key and despite white returning it everytime, he insisted that white should keep it because he was waiting for black to come back to him again. Even in his dreams, white approached him calling him "ikeda" but when he held him, Riya called him "Atsushi" and him smiling and blushing as he sleeps says "so he turned into black" before he woke up. the one he said "I love you" to is Black. He hesitated sleeping with him because of what Black had said. "What if I disappear too" He said he didn't mind which ever personality it was as long as it's riya but he is clearly afraid that black will disappear while He didn't seem to be that worried when White did. And when he slept with black and woke up to Riya being White. He was clearly lonely. He was jealous because now that Riya is white, he was afraid Riya would come back to that senpai. Because he knew White is in love with the senpai. And white Riya laughed when he asked him if he loves the senpai. Before white riya leaves, he notice ikeda looking depressed. And white knew Ikeda is sad that Black is gone. So white says dont look like that. Black is still inside him. Ikeda wasn't this devastated when it was white who disappeared. White even thanked Ikeda for loving Black.
Ikeda Atsushi kept saying til the end that he is fine with either black or white but when they saw each other again years later, He still asked if the Riya he is talking to is Black or White. If he never really cared whoever personality it was, why bother asking. Deep inside he was hoping that Black returned to him. And Riya saying black and white is combined into one riya now made him happy.
And riya calling him atsushi makes me think that the part of him with feelings for Ikeda Atsushi will always be Black.