The sentence that contains a metaphor is: "The sunset was a masterpiece of brilliant oranges, pinks, and purples."
In this sentence, the sunset is described as a "masterpiece," suggesting it is beautiful and skillfully created, rather than literally being a work of art.
(Note: The sentence "I was so hungry that I could have eaten ten thousand hamburgers" contains hyperbole, which is an exaggerated statement, but not a metaphor.)