Munou by österreich: An Analysis
Beforehand note: The song doesn't have to refer to anything. It's just me interpreting the lyrics since I believe that Ishida was the one who chose this song as the OP (since he did make a tweet about it and he does like österreich a lot).
For example, "I had a dream" can refer to Kaneki as a whole up until he stopped being Haise and remembered the truth about his mother ("It was of the time we were born."). "We lived such a beautiful life" fits as well since it is referring to how Kaneki put the wool over his own eyes and pretended to be kind and selfless.
Now, "in a city of wombs" is very complicated. It can refer to the notion of the Wombs of the Washuu clan, since Kaneki's other personality takes on the form of Rize early in the series (very subtle foreshadowing to the fact that Kaneki was messed up since before the series).
"We clapped our hands and smiled / We drank the paint dry / The colors of the rainbow changed". From what I think, it can be literally be saying how Kaneki was so desperate to be the "kind and selfless" person he is that he "drank the paint dry," as in his facade fell apart and they (well technically one at that point in the manga) realized his true face. Thus, Touka punched him and yelled at him (the colors changed).
"We started wanting to be held / I couldn't walk alone anymore." = Kaneki's insane fear of being alone. May also be his want to be held in the arms of death that comes later on (makes a true and not-so-subtle first appearance in chapter 140-something).
"We gave our love to toys" Kaneki likely viewed his "friends" as toys he played with to keep himself from being lonely when he first met them. These feelings dissipated by the time :re rolled around.
"The prostitute spat fire." Prostitute=Rize (in a way, considering what she was born to do). Rize also "spat fire" in the fact that she successfully ran away from the Sunlit Garden and spat in the face of V/Washuus.
"In a faraway city, someone died" can refer to Kaneki's death as an imposter (becoming Akaneki or Yamineki), Shirazu's death (he died far away from reaching his goal), or Arima's death (he died far away from the world he hated, the world were he was forced to take lives again and again).
Some of the stuff I skipped is basically redundant words as it just elaborates on some lines here. Hope it makes sense!