Depends entirely where the sniper intends to use it.

If you’re interested, look up Carlos Hathcock, a USMC sniper during the Vietnam war, and considered one of the best snipers for a very long time. I don’t recall the details offhand but one of his confirmed kills was a high ranking military officer. He spent something like 3 days in a gillie suit crawling something like 1000 yards across a field, took out his target, and successfully evaded troops searching for him that walked within feet of his location.

Keep in mind he's thousands of feet in the distance. Up close yeah it's fairly obvious he's not a real plant, but as a speck on the horizon you're fukt

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.

An old favorite helps to answer this question:

https://www.theguardian.com/artanddesign/gallery/2014/feb/19/sniper-camouflage-simon-menner