Do all the xenos have red blood?

[Morgrim]

Between the annelid and crustacean animal groups, blood pigments can be red, green, blue, yellow or purple. I figure if we can get those colours on earth, could certainly get them in xenos; the oxygen carrying compounds are known to exist.

For some reason I've always visualised tyrannids as bleeding a greenish-yellow pale ichor.

[Madjob]

Ork blood is definitely red. It was stated for certain in a white dwarf.

The green colour in their skin comes from chlorophyll so they can photosynthesize sunlight like a plant but their physiology underneath is mammalian like us.

This means their blood is red, plus it looks more dramatic on their green skin.

Actually it's because the spores in their blood can grow into a thin algae membrane just underneath their skin.

Though both explanations are not mutually exclusive.

[Asymmetrical Xeno]

my enslavers blood is yellow and glows...

[The.Justinian]

Tyranids don't neccessarily have blood at all. Depending on the environment, the Hive might reeingineer the metabolic process of its creatures for an anaerobic environment, instead using chemical-voltaic storage (yes, battery powered tyranids); nuclear fuel cells, or who knows what.

I've always imagined the big tyranids that are obviously war-beasts designed from the ground up (warriors and gaunts appear to have been something other than soldiers at some point in their existence)...are powered by something other than eating. Their abdomens are far too small to support digestion for their energy level, and that kind of digestive system would be redundant weight in combat (never seen a solar attack helicopter or MBT, have we?). So Carnifexes, Trygons, and Tyrands thus likely carry predigested energy...just imagine rippers puking some nutrient solution down their gullet, or perhaps a digestion pool beaming them microwave power between battles.

How the power is circulated, however, to the muscles and brain would have to be through either a bloodlike liquid (pumped by a heart or very toned, redundant arteries), or through wiring (more believable than one would think, as biometals in their skeleton or carbon fiber are the only things that could make them as tough as they are). However, this is merely the conduit for nutrients, and needn't necessarily be carrying the oxygen as well. Many terrestrial lifeforms have a nutrient-circulation system but use alternate means of aquiring oxygen.

Most of the assumptions in the posts above have assumed aerobic respiration+metabolism of sugars as the basic means of cell function, and we'll play along and assume that even if the big beasts are carrying a nuclear fuel cell, it's still assembling sugar for the rest of the body, and will need an oxidizer (oxygen for today) to get the energy back out in a biologically relevant form.

They could acquire this oxygen not from blood but through small respiration tunnels throughout the surface of the organism, or could store pure O2 in bladders near the muscles for bursts of energy. In insect wing muscles, their surface is so close to the air that much of the oxygen is garnered through diffusion (also one of the gold medalists of the animal kingdom for most efficient aerobic energy conversion). Remember, we're dealing with an organism that's designed deliberately, rather than having the legacy of being evolved from a slug (which we, unfortunately, do have to deal with every day).

Do tyranids have circulatory fluid? probably, unless they're close to cyborgs with bio-wiring (possible but wouldn't sit well with most). Does that fluid have a color based on the oxidized metal that is carrying the oxidizer for metabolism? Not necessarily. It depends on whether you think your bug has lungs.

I'd say that given the size of their thorax, that's what's likely is in there...one GIANT lung, gathering and concentrating O2 for the body. The energy is simply stored, but the oxidizer needs to be acquired. So what's the color of it? probably the color of the easiest to replace/assemble/most efficient compromise element on this world, subject to change. Given that copper requires less energy to smelt into a useable form on our terrestrial earth, the hive mind would likely use copper unless it was far less efficient than Fe. Thus wounded carnifexes could get a cheaper/more available blood transfusion as they were rehabilitated for battle.

So...
If Tyranids were to invade a planet with a similar mineral profile to earth, with a similarly oxygen-rich atmosphere, their blood would be...
GREEN.

[Valkerie]

Ork blood is definitely red. It was stated for certain in a white dwarf.

The green colour in their skin comes from chlorophyll so they can photosynthesize sunlight like a plant but their physiology underneath is mammalian like us.

This means their blood is red, plus it looks more dramatic on their green skin.

What he said.

In an older White Dwarf,[and I can't remember which one now,] one of the designers said something to the effect that Ork blood was supposed to be green, but that when painted on an Ork it just looked like they'd been sick all over themselves, so they painted the blood red so it would look better.

[gilbert93dt]

I tend to make the blood of my Tau blue. Only because it looks awesome with their golden armor and white trim. It essentially acts as the much needed third color.

[lobster-overlord]

What he said.

In an older White Dwarf,[and I can't remember which one now,] one of the designers said something to the effect that Ork blood was supposed to be green, but that when painted on an Ork it just looked like they'd been sick all over themselves, so they painted the blood red so it would look better.

I remember the same article. It also referenced that because we bleed red, that our minds don't really process other colors in as gory a manner as with red.