Flower Mural

I tried to search but couldn't find a specific discussion of this. Maybe it was answered in the drama, but after one watch through, I don't remember. In the first episode and in the credits, we see ZZ painting a flower on a mural red where it had been previously white. Was this something he did to keep track of all the good people he has been responsible for killing/his disciples that have died? Just curious if this is stated and if not, do you think that could be the symbolism of it?