There are two genders in Danish; common gender and neuter gender. For common gender nouns like duck; "and", the article for the indefinite form is "en" so "a duck" becomes "en and". "Æble" is neuter gender and uses the article "et". "An apple" is "et æble" :) it has nothing to do with a/an in English by the way, which only changes according to the following sound of vowel or consonant.
My problem with this was with the lady who says the words. I listened to the sentence 10 times, listened to it slowed down another 10 and still thought she was saying grøn æble. I'll have my fiance listen to it when he gets home as he is a born Dane and has corrected the pronunciation and even some words and phrases before. But with the poor quality of the sound bites, this is a reoccuring problem.
The "y" in yellow is here as a consonant, not a vowel, so it is "a" yellow duck. If it were "old" for instance, it would read "an old duck" because "o" is a vowel. Y can be tricky for English learners because it can sound like a vowel, and is considered a "helping" vowel at times, but at the beginning of a word, think of it strictly as a consonant.