The USDA says "iridescent beef" is perfectly okay.
People are like moths to the flames that are rainbows. The next time there's a rainbow outside, notice how many people drop everything, even important things, to Instagram it. It would be a perfect time for aliens to take over Earth. Rainbows where rainbows shouldn't be, however, cause alarm. Take beef, for example.
How many times have you (if you eat beef) foregone a package of sliced roast beef for a different package because said beef was slightly iridescent? If you don't eat beef, perhaps you've seen a package of said rainbow meat and it reminded you why you no longer eat it. The Internets are clogged with threads like, "Why does deli roast beef look like a rainbow?," and the ever gravid concern, "Subway shiny roast beef?" And while everyone should be spared urbandictionary.com's definition of what a "Beef Rainbow" is, the truth of the matter is, there's nothing inherently wrong with rainbow meat.
Beef rainbows aren't a sign of spoiled, tainted, or (sorry) magical beef. There's enough speculation over the integrity of rainbow beef that the USDA's website has a section on "Iridescent Color of Roast Beef" near similar topics like "What does 'natural?' mean" and "what is beef?" According to the USDA, "When light hits a slice of meat, it splits into colors like a rainbow." This is something called a "diffraction grating," essentially what happens when light waves bend or spread around a surface and create a pattern. It's the same thing that happens to make rainbows on the surface of a DVD. It's understandable that folks mistake diffracted light as a sign of spoilage, especially since the main color created by meat diffraction gratings is green. There is a reason why in Dr. Seuss's Green Eggs and Ham, the central conflict of the protagonist is his strong apprehension against eating green meat.