There are lots of answers throughout the course that aren't accepted but are perfectly acceptable in real life situations.
So does the cat wear 2 pairs of shoes is that the problem and why wouldnt this need to be identified with 'Hai"
đôi means a pair i believe, so in English we can just ignore the pair portion and say "blue shoes"
it is very common for cats to wear shoes in Vietnam. so this is a every day saying.