Polish adjectives and nouns change. This is called declension. Nouns change with cases (there are 7) and number (singular or plural), Nouns also have grammatical gender.
Adjectives have to match nouns- they change with gender, case and number.
If the noun is a subject of the sentence it is in case called nominative=mianownik. czarne skarpety
The object of verb mieć is in case called accusative. As it happens for "czarne skarpety" , accusative = nominative. So "Masz czarne skarpety"
If the verb is negated, instead of accusative you need case called genitive=dopełniacz,
Nie masz czarnych skarpet.
It seems that every new word we have to guess what the endings might be! Some of the colours were ending in ej on this recap exercise but this seems to be the only one that is (czarn)ych and I keep on making the same mistakes each time the words change. It's very frustrating. I can happily write most of the sentences but never know which endings to use. Skarpet skarpeti skarpety and so on, it's the pies psa psem situation again, having to learn by trial and error... however the explanations are helpful and the lessons are quite fun!