I've had this happen with a number of packs. I sweat a lot. Newly dyed fabrics don't always retain all their dye when exposed to warm salty sweat. Like Nick, I had my original spinnaker fabric GG Murmur do this, also with EcoMesh shirts as it happens, which are my favorite 3-season hiking shirt. The leaching was restricted to the back of the hiking shirt and top of hiking pants, where typically black mesh fabric either made up a pad pocket (GG) or covered a frame sheet or other pack element in another manufacturer's pack. GG seems to have fixed this problem.
Eventually any dye leaching stops and eventually the discolored areas on hiking shirts also fade, though not completely. But they are, after all, backpacking attire. Stains are an indication of the trail miles they have seem, a mark of "character" if you will. I love GG stuff, and own a lot of it. I still use my old Murmur, which no longer has spare dye to stain anything with. I still use my EcoMesh shirts, which are now conveniently pre-stained should a new pack tend to leach any dye. ; )