This qualitative multiple case study explores the collaborations between three STEM middle school teachers and three STEM undergraduate mentors of color in an urban school district. Drawing on sociocultural theories and literature on culturally relevant education, we used a comparative thematic approach to explore how mentors contributed to culturally relevant opportunities in STEM curriculum and pedagogy. We found that the partners' STEM identities, how the teacher positioned the mentor in the learners' experience, and the teachers' philosophy of the purpose of engineering influenced the contribution undergraduate mentors could make to rigorous and equitable engineering instruction.