The group plans to highlight findings at a news conference Tuesday that it says show young people of color have faced a lopsided number of arrests for low-level marijuana crimes.
The measure on the November ballot would let adults possess up to an ounce of marijuana for personal use.
Residents could legally grow small marijuana gardens, and individual cities and counties would decide whether to allow marijuana sales.
A Sacramento preacher who is leading opposition to the measure says he is outraged by the NAACP decision. International Faith-Based Coalition president Ron Allen says illicit drugs harm the black community.