From my understanding it’s easier to use brown as a base color and add natural foliage to green it up than it is to have a green base color and add dirt to tan it up.
Brown is a lot more common close to the ground than you might think even in summer but it’s especially useful when you consider using the suit in fall and early spring.
Ghillie suits are meant to make you vaguely look like a grassy blob. Humans are really good at picking out the shape of a human head/shoulder area so the suit makes that disappear.