That strange, colorful shimmer on your deli ham is usually just physics at work, not rot. Light hits the aligned muscle fibers of thinly sliced meat and splits into a rainbow, much like a CD or soap bubble. Add curing salts, moisture-retaining additives, and razor-thin slicing, and that iridescent glow becomes even more dramatic—yet still perfectly normal and safe.

The danger starts when the rainbow is joined by real red flags. A gray, brown, or green cast, fuzzy spots, or an excessively shiny, slimy surface all point to bacterial growth. A sour, sulfurous, or ammonia-like odor means the ham belongs in the trash, not your sandwich. Store deli meats tightly wrapped, in the coldest part of your fridge, and eat them within a few days of opening. If everything smells clean, feels firm, and only the rainbow looks strange, it’s fine to eat. The moment your senses disagree, walk away.