Harp

The harp is unique in orchestration, belonging to no specific instrument family, yet still part of the standard orchestral layout. Its sound often gets lost when accompanied  by the other instruments, so it frequently plays solos. 

Similarly in Harp, the strings appear colorless—or even vanish entirely under certain light conditions—until you interact with them. As you ascend the stairs, only the strings nearest to you become vibrant with color.