Why does deli roast beef look like a rainbow?

The iridescence of meat products is produced by a combination of the angle of incidence of the light on the muscle fibers and the wetness of the surface. The sharper the meat slicer, the more likely iridescent deli meat is. It doesn't mean the meat has gone bad or that it isn't safe to eat.

It's the breakdown of the proteins due to the chemicals they put in it to prevent spoilage. The chemicals break down the proteins in the process. You see it on ham as well. You would never see this in a piece of meat you actually roasted at home. Of course store bought roast beef isn't actually roasted. You are correct it's gross.

B/c of the oils and greases inside the meat

good question but i think its from some kind of oil or something ut on it to keep it fresh longer