Why is there a luminous orange hue in this piece of tuna?
From Cooks Illustrated


  The rainbow effect is caused by the reflection of light off muscle
  fibers, technically known as double refraction or birefringence. It
  occurs when the muscle fibers are cut crosswise and can be observed
  for several days after slicing the meat. Light striking the ends of
  the fibers is reflected in two different directions, appearing to the
  eye as a rainbow of colors. Just like a real rainbow, the multicolor
  appearance is fleeting, visible only when the muscle fibers are cut at
  a specific angle relative to the grain of the meat and when the meat
  is viewed at a certain angle.
  
  One more point to keep in mind: While the rainbow effect is harmless
  and doesn’t offer any indication of how fresh the meat is, there is
  another color change that is a sign of meat that has passed its prime:
  the development of a green pigment. Meat with a green cast is
  contaminated with bacteria and should be avoided.