As Haynes and his band, aided tonight by the Colorado Symphony, break into their set opener, the 1968 classic “Dark Star,” a cheer erupts from the thousands of Rocky Mountain Deadheads in attendance. This initial loud welcome is quickly followed by another, and possibly even louder, ovation, as the crowd acknowledges the instrument strapped across Haynes’ torso. It’s a guitar so mythologized in Grateful Dead lore that it’s known by just a one-word name: Tiger.