I picked the right answer but why don't we need to add "màu" before the color red?
I think because it's acting as an adjective. Màu is the classifier, so we only need it when it's a noun. I think.
What's the differemce between chi ay and co ay?
In a normal conversation, that can be used to distinguish the age (chị is younger than cô). Moreover, cô also means "Miss/Madam" in someway