Red is a symbol of death.
From the moment Doodle is born, he is expected to die. Preparation involves building a coffin rather than a crib. He is also described as red, a symbol of death.
He seemed all head, with a tiny body that was red and shriveled like an old man's.
Doodle is also the first one who spots the scarlet ibis, which is in the “bleeding” tree.
He slipped out into the yard, and looked up into the bleeding tree. "It's a big red bird!"
The red imagery is again connected with death, because the bird will die and Doodle will die.
Red as a symbol of death is common, because we naturally equate red with blood and often with death. This is why the ibis is “scarlet” and the ibis itself is such an important symbol throughout the story. Doodle seems to know he will die, even after everyone else has accepted that he will live.