In the book "Beloved" by Toni Morrison red is symbolic of death and violence. he novel is about American slavery and former slaves lives during the period of reconstruction. Sethe escapes to a neighboring state that does not allow slavery. She gives birth to a child during her escape. When the slave hunters go to take her back she goes temporarily insane and slaughters her baby leaving blood everywhere. The act of the baby's death is so horrendous that the men leave. Her deceased child haunts the home and eventually returns in the form of a young woman.
Baby Suggs is Sethe’s mother-in-law. She was present when Sethe went crazy and committed infanticide. She was a witness to the bloody aftermath. Stamp Paid knows that for Baby Suggs to ponder red only brings back terrible memories.
Basically because the color red represent blood and love. All the things Baby Suggs couldn't have and/or shouldn't remember.
Also, remember during the Misery, the bloodshed that happened made Baby Suggsrealize that she can't help people anymore if she can't help the one who is most like family to her.