Blood

Blood is a combination of certain types of specialist cells suspended in a fluid which is found in most corporeal lifeforms. Blood circulates the body delivering nutrients to cells and taking away waste products.

Depending on the base material of the blood it can be different colours for different species. Many species, including Humans have red iron based blood. Vulcans and Romulans have green copper based blood, whilst Andorians and Bolians have blue blood. This often results in cultural significances of the colours, for instance on Human starships a red alert signifies danger, whilst on Romulan ships red is a good omen with Green lighting indicating danger. (ENT novel: The Good That Men Do)

Blood cells are some the first to be assimilated when an individual is injected with Borg nanoprobes, the nanoprobes find blood cells and latch onto them taking control, this process causes the grey veiny appearance which signifies someone has just begun the process of being assimilated. (VOY episode: Scorpion; TNG movie: First Contact; et al.)

Ancient Vulcan tradition hold thats dried blood seasons the blades of weapons. (EV comic: Cloak and Dagger)