In the final paragraph of "How It Feels to Be Colored Me," Zora Neale Hurston explains why she calls herself a "brown bag of miscellany." She says that all the people in the world are like bags of different colors, each of which contains a wide variety of objects, "priceless and worthless."
This image makes two points. First of all, it is impossible to tell from the color of the bag what is inside (meaning that racial prejudice is absurd). Second, everyone has a mixture of good and bad, or treasure and junk, inside them, and people are approximately equal in this respect. No one is vastly more or less valuable than anyone else. This is another reason why all types of prejudice, including racism, are foolish.
In 1928, when Hurston wrote this essay, the expression "a mixed bag" was a fairly new one. It appears to have originated among hunters, who used it to refer to a hunting bag filled with different types of birds, some of which were more prized than others. The term could refer either to a diverse group of people or to the contrasting qualities within a single person. Hurston's extended metaphor is one of the earliest and most vivid expressions of the latter idea.