Yellow-6

Yellow-6 is a toxic chemical produced by Umbrella and is one of the key components of V-JOLT (also called UMB-20), which is a powerful herbicidal compound that has the strength to kill the majority of mutant plants such as Plant 42 or Plant 43. The product was most likely used to create many other experimental compounds, but when found on-site in the Spencer Mansion pharmacy, it was stored alongside other chemicals which could be used together to create V-JOLT, along with a handful of other incompatible ones (used for unknown purposes). It's a yellow-coloured liquid stored in a glass bottle.

Interestingly, an unrelated Yellow 6 is also a real-life substance otherwise known as Sunset Yellow FCF (E110) or FD&C Yellow 6. It's mainly a colourant or food additive and its use in food and cosmetics is controlled, but you'll usually find it mixed with other colourings in Chocolate or Caramel in order to achieve a more vibrant colour. It most likely got its name as it was the next artificial yellow food colouring certified by the US FDA after Tartrazine, otherwise known as FD&C Yellow 5.

In Resident Evil, the name is merely for 'flavour', and the number is the most important aspect. The player has to combine chemicals until they 'add up' to the number 20, which creates the V-JOLT solution. For example, other chemicals include UMB-3 and VP-017.

In the Resident Evil Remake, the bottle (and all other bottles for that matter) have the label 'Acid Nitric'. Nitric Acid (or HNO3) is a very reactive mineral acid reagent commonly used as an oxidiser for liquid fuel rockets, or as an etchant, or in fertilisers. It is also devastating to most organic matter including skin, where a chemical burn happens instantly and also stains the skin yellow.