Do all the xenos have red blood?
Do all the xenos have red blood?
Probably not. No.
Tau, for example, have blue blood.
Tyranids can have just about any color blood you can imagine, most likely.
Nope, as I am sure you well know human blood only appears red because it uses iron as a "bonding agent" to transport oxygen around the body, when iron bonds with oxygen (oxidizes) it turns red, when we see this in the world around us, we call it "rust." In actuality human blood is mostly plasma (i.e. water) and fairly colorless. All it would take for xenos to have different colored blood is for it to use a different metal to bond with the oxygen, thus Star Trek justifies Vulcan blood as green because it is copper based.
Tau blood is blue I believe, possibly cobalt based (though I'm not sure).
Ork blood includes chlorophyll and copper, so it might well be greenish.
The Kroot have seperate circulatory systems, the most basic and integrated of which of which is Ork based (the consumption of Orks from a wrecked ship in the dim past seems to have been the catalyst for what they are now), while the other could be different depending on what the kindred has been feeding on.
Tyranid blood uses bonding compounds unfamiliar to the Imperium, so it could be any color. Genestealer blood may be similar to base creature's blood. Also, given the hivemind's propensity for tinkering with it's species, their circulatory system could be re-arranged in any number of ways, so no way to predict its color.
Some species (like the Q'Orl) have something that isn't really "blood" but fulfill a similar role, while still others (e.g. Umbra) have nothing even remotely recognizable as blood.
In short, Xenos blood could be any number of different colors, depending on their specific biology.
EDIT:
Aaagh! Why would you even joke about starting that discussion again!?
Cry woe, destruction, ruin, and decay:
The worst is death, and death will have his day.
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.