fun fact: it's actually not the red color of the cape that makes the bull attack the Matador, but it's movement. The cape is only red so you don't see the bull's blood on it.
Right now Yuu is blooming with her love but once a flower has bloomed it shrivels up and dies. I think this is the metaphor for what represents Yuu's and Touko's love. The front cover of chapter 19 is like a mirror. When one person’s love has blossomed the other person’s love dies. As Yuu's love is blossoming Touko's love will die. Also the name of chapter 19 is mirage, the definition of mirage “an unrealistic hope or wish that cannot be achieved.” Basically, the author is saying that a happy ending will not be achieved that’s what I get from this whole manga. :(