To become slightly brown or dark in color.
When something subfuscs, it means it gets a little bit brown or dark.
Examples:
The banana starts to subfuscs when it gets old.
The leaves subfuscs in the fall.
The bread subfuscs when it is toasted.
The apple subfuscs after being left out in the sun.
The paper subfuscs when it is burned.
The paint subfuscs when it dries.