This volcanic chain is a result of subduction of small microplates beneath the North American Plate. Medicine Lake Volcano is a young shield volcano which began forming about a million years ago. Common surface rocks in this region include basalt, andesite, dacite, and rhyolite. Because these rocks cooled quickly they are fine-grained in texture. Basalts tend to be light to medium gray.
Andesites are dark gray to black.
The dacite I've seen is gray with flecks of darker colors in it or almost black with lighter flecks. Rhyolite is light-colored unless it is obsidian. In addition to the varying composition, rocks have different expressions depending on the circumstances surrounding the rock's formation. When a rock cools so fast that no crystalline structure is developed then the rock is called an obsidian , which is typically rhyolitic or dacitic in composition. Satellite imagery is affected by the composition of the rock as well as its form.