Translation:The boy wears multicolored pants.
If the sentence is "Το αγόρι φοράει πολύχρωμο παντελόνι," then the translation would be "The boy wears multicolored pants." If the sentence is (as in the example) "Το αγόρι φοράει ενα πολύχρωμο παντελόνι," the translation should be "The boy wears a pair of multicolored pants." If "ενα παντελόνι" is used, it should be translated to a pair of pants"
In Greek this distinction is not as important as in English. The two phrases are more or less interchangeable. The word "ένα" can be omitted if it is not the focus of the sentence the same way you omit "εγώ" in the phrase"[εγώ] τρώω".
I think Duolingo added the word "ένα" to show you can add it if you want. Actually without it the phrase sounds more natural.
The word "ένα" would be necessary only if the Greek phrase implied that the boy wears only one pair of trousers and not more. Without it the phrase is more like a casual observation of someone's clothing.
But in the current case I do think both "The boy wears multicolored pants" and "The boy wears a pair of multicolored pants" should be accepted as correct translations. It should also accept "The boy wears one pair of multicolored trousers".
It's basically the same thing, but I'd say there's a higher chance of someone calling a painting "colorful" instead of "multicolored", for example. It depends on the word that follows, sometimes "colorful" is preferred to "multicolored" and vice versa. "Colorful" also has the meaning of "vivid", whereas "multicolored" does not.