Pigments differ a lot in their natural opacity: deep reds, yellows, and oranges in particular use pigments that are naturally more translucent and need extra layers of paint particles stacked up before they fully block the color underneath. It's also harder in general to cover a dark existing wall with a lighter new color than the other way around, since the underlying pigment has to be completely overwhelmed rather than just tinted. Using a tinted primer matched roughly to the new color can cut the number of coats needed significantly.