The purpose of this study is to learn about the effects of an experimental treatment called RO7198457. This study is for patients with Stage II or III colon or rectal cancer, which has been treated surgically and with chemotherapy, and your blood has been found to have ctDNA (circulating tumor DNA) during a ctDNA screening test. RO7198457 (autogene cevumeran) is an immunotherapy that is individualized to a tumor and designed to mount an immune response against it. Treatment includes RO7198457 being injected in the vein, physical exams, blood sample collections, and CT scans for up to a year. Risks include fatigue, fever, and headache. Patients may or may not receive direct benefit while on the study, however, information collected during this study will help people with colon and rectal cancer in the future. RO7198457 is considered "experimental" because it has not been approved by the U.S. Food & Drug Administration (FDA) for the treatment of any disease.