The first time I really had a good look on the wedding page of the picture book was episode 18, when Hiro talked about the ceremony with the group.
The first thing I thought was that the princess and prince in the picture looked like Kokoro and Mitsuru, because of their hair color.
Then I just forgot about it.
Until I saw the ending with a little nu-Hiro reading the picture book. How much more significance could the storybook have if the princess had pink hair instead of blonde? nu-Hiro could simply remark 'Hey! You look just like the heroine in the storybook!' But no.
Now, this is just a personal thought, but what are the chances the current generation would associate the picture book with Kokoro instead of Zero Two? Yeah, I'll stop here, not sure where this train of thought is heading lol.