Blood

Blood was an essential fluid of many life forms, Gallifreyans and Humans included.

Food
Some organisms consumed blood in order to survive, namely Vampires, Haemovores and Plasmavores. (DW: State of Decay, The Curse of Fenric, Smith and Jones)

On their home planet, the Ogri subsisted on amino acids, but found that the blood of Earth life forms, including Humans, would also suffice. (DW: The Stones of Blood)

Other species, such as Vortisaurs and Androgums did not subsist off of blood exclusively but would consume it as part of a more varied diet. (BFA: Storm Warning, DW: The Two Doctors)

Practical Use
Mortimus used the blood of five species whose destinies the Time Lords had affected in a ritual to summon Artemis, the Chronovore. (NA: No Future)

Blood also formed an important part of some Faction Paradox rituals. Sometimes biodata would be extracted from the blood of a being to use in a Faction Rite or Ritual. (EDA: Alien Bodies)

Some species, including the Sycorax, used a form of hypnosis known as blood control. (DW: The Christmas Invasion)

Judoon scanners checked blood to gather information on the units species. (DW: Smith and Jones)

Colour
There are many variations in blood colour. This may have been the blood of the child who was harvested by the 456
 * Human and Time Lord blood was red.
 * Many insectoid species, including Mayflies had yellow blood.(TW: Reset (TV story))
 * The Judoon had yellow blood. (SJA: Prisoner of the Judoon)
 * The Ood had purple blood. (DW: Planet of the Ood)
 * The 456 had red blood. (TW: Children of Earth: Day Five)
 * Sontaran, Foamasi and Meep blood was green. (DW: The Two Doctors, The Leisure Hive, DWM: The Star Beast)
 * The Waro had yellow-green blood. (PDA: The Devil Goblins from Neptune)
 * The Nostrovite has black blood. (TW: Something Borrowed)
 * Kaled blood was Green. (DW: Revelation of the Daleks)