The orange part of the diagram is which set? (′ means the complement within U.)

Everything outside circles A and $B$ is orange: the area outside all three circles and the part of $C$ that is in neither A nor $B$. That is $(A\cup B{)}^{\prime }.C-(A\cup B)$ leaves the outside white, and $(A\cup B\cup C{)}^{\prime }$ leaves the C-only part white.