Guinness looks black because of how the barley is roasted, but technically it is a very deep ruby red rather than pure black.

The science in a sip

  • Guinness uses barley that is roasted at high temperatures, similar to how coffee beans are roasted.
  • This roasting makes the grains very dark and creates pigments that give the beer its deep, almost black appearance in the glass.
  • When you hold a pint up to strong light, you can see that the liquid is actually a dark ruby red, not true black.

Why it looks black in the pub

  • In normal bar lighting, that dark ruby color is so intense that the human eye just reads it as black.
  • The contrast between the dark body of the beer and the creamy white head from nitrogen bubbles also makes the dark part look even blacker.

Quick forum-style takeaway

Guinness is “black” because roasted barley turns the beer an extremely dark ruby red that looks black in typical lighting, especially under that creamy white head.

TL;DR: Guinness is not truly black; it’s a very dark ruby red created by heavily roasted barley, which appears black unless you shine strong light through it.

Information gathered from public forums or data available on the internet and portrayed here.