Up in the mountains, at high elevations, the air is dryer, the UV rays are stronger, there’s no mold, mildew, algae, or fungi to attack the deadwood to induce rot. So, yes, the deadwood is white, naturally. (Or a very light grey.)
When we bring the collected trees down from that environment, the deadwood is now subjected to all those potential rotting elements, and it will start to rot! Lime sulfur is used to help prevent the rot, and it dries white, so it has the same look as the tree had in the mountains. There is some manner of skill necessary to make the line sulfur look natural. If painted on straight on dry wood, it will just look like white paint! The technique I use is to wet the deadwood, then let it dry out for about an hour. Then I apply a 50/50 mix of lime sulfur/water using a paint brush. The lime sulfur tends to absorb into the wood. When it dries, it won’t be as stark white, and the texture is the wood grain will still be evident.