The red was painted by ISIS goons. It’s their shorthand for “Christians” and is meant to mark Christians for death or expulsion. The black was written by Muslims opposed to ISIS. It means “We are all…” (Remember that Arabic is read from right to left).
UPDATE: It appears the image is photoshopped. Don’t know by who though. In the meantime, the good news remains that there are Muslims in Iraq opposed to the persecution of Christians.