Why is Slepice jsou žluté wrong
"Slepice" is a "hen" and I don't think "slepice" is a valid translation in this sentence.
Chicken is very much used as a synonym for hen (a Google image search for "chicken animal" yields almost exclusively adult hens). Kuřata are babies and are called "chicks". So if anything I feel slepice is more accurate than kuřata.
That is correct and in other sentences "slepice" is accepted for "chicken" but the meaning of the sentence "are yellow" shows the speaker probably really meant chicks.