Many people know that our blood is red because it is iron based. Many people also know that blood is blue or green if it is copper based. I'm wondering what color blood would be if it was let's say aluminum or tin based? How could I achieve white or black blood?

Colors I'm curious about

  • Black
  • White
  • Orange
  • Brown
  • Yellow

Metals I'm curious about

  • Tin
  • Aluminum
  • Platinum
  • Gold
  • Silver
  • Cobalt
  • Nickle
  • Zinc

Don't worry about the effects or problems of the colors or metals that will be a separate question.

  • 1
    $\begingroup$ I assumed something about chromium, did a little research and found out I was completely wrong. This may provide some answers for you. The chemistry involved is a little over my head, so gave up trying to decipher. Hopefully you'll have better luck. Also, since gold doesn't oxidize, it wouldn't work as a base for blood, unless your world didn't use oxygen as the catalyst like we do. Silver oxidizes black. I believe aluminum oxidizes white. $\endgroup$ Oct 24, 2015 at 21:29
  • $\begingroup$ Could you accept that blood stays red, but that skin has changed to block certain wavelengths of light? A lot of people believe we have 'blue' blood, because skin due to light wavelengths being filtered differently by the skin. This depends on your story, so I just wanted to ask if we could take this alternate approach. $\endgroup$
    – Mikey
    Mar 28, 2016 at 19:03
  • $\begingroup$ @Mikey Since I'm looking for a wide variety of answers (and choices), not only do I think you should expand on this idea, I highly encourage it. $\endgroup$
    – TrEs-2b
    Mar 28, 2016 at 20:28
  • $\begingroup$ @TrEs-2b - I tried for a bit, but couldn't get both White and Black - White is 'All Light' or 'No Pigment'. Black is 'No Light' or 'All Pigment'. For folks to be walking around with Black Blood, the skin should block all wavelengths to your veins. For White Blood, it must allow all colors to reflect. I tried at length, but couldn't get you an answer that is around "skin" properties. Just a thought - happy writing! $\endgroup$
    – Mikey
    Mar 28, 2016 at 20:53
  • $\begingroup$ Perhaps this answer could help. $\endgroup$
    – Wyglaf
    Apr 6, 2016 at 13:51

6 Answers 6


There are many proteins that transport oxygen in the terrestrial biosphere, this, and this.

One thing to note is that at human body temperature, atmospheric pressure, and Earth's atmospheric concentration of oxygen Hemoglobin is by far the most efficient oxygen transport protein among the list included in this post. It varies from 50% more efficient to 10x more efficient than the other oxygen transport molecules listed here.

enter image description here


Hemoglobin uses iron as its metal atom. We are all familiar with hemoglobin and its color red.


Hemocyanin's can be found in certain mollusks. It is colorless when not transporting oxygen and blue when transporting oxygen. Hemocyanin uses 2 copper atoms in its protein structure and the protein is very similar (but not identical) to hemoglobin.


Chlorocruorin's uses $Fe^{II}$ as its metal ion.

A dichromatic compound, chlorocruorin is noted for appearing green in dilute solutions, though it appears light red when found in concentrated solutions.


Hemerythrin uses $Fe^{II}$

Hemerythrin is an iron bearing oxygen transport protein often found in the muscles of marine invertebrates.

Hemerythrin and myohemerythrin are essentially colorless when deoxygenated, but turn a violet-pink in the oxygenated state.


Erythrocruorin is an iron bearing protein. I didn't find definitive description of its color but I think it would be a light red.

Erythrocruorin is a large oxygen-carrying protein, whose molecular mass is greater than 3.5 million Daltons. It is related to the similar chlorocruorin. It is found in many annelids and arthropods (including some insects).

and Earthworms.


Only seen in the mollusc Pinna squamosa. Brown manganese-based porphyrin [oxygen transport] protein.


Coboglobin is currently a synthetic protein that performs the same function as hemoglobin. It uses Cobalt in place of the iron atoms.

Blood of this type would be amber yellow in color when in the veins while uncoloured and clear in the arteries.

It might make an interesting blood for synthetic biological organisms (like the one found in the movie Aliens)...

Chloro-carbonyl-bis(tri phenylphosphine)-iridium

An Iridium (one of the Platinum Group Metals) ion based molecule that is yellow when oxygenated and dull orange when not. It provides the additional capability of serving as a hydrogen transport molecule too - which might make for interesting biology on a gas giant planet. The organism would require a biological/chemical pathway to generate oxygen from chemicals found in the environment and then transport that molecule to the cells that needed energy. Perhaps the oxygen circulatory system and hydrogen circulatory system would need to be separated?

In solution, the compound takes up one atom of oxygen per molecule to change from brilliant yellow to sullen orange...

In the oxygenated condition, the iridium-based blood of extraterrestrials would have to be protected from light because it is very photosensitive. The pigment slowly decomposes over a period of days or weeks when exposed to strong light, gradually changing color from orange to green and finally to a deep bluish-black. Such aliens would therefore either have very dark skin, or would inhabit a dimly lit world. (In the absence of light, the molecule is stable for years.)

The iridium complex has one additional property which is extremely fascinating to xenobiologists. In addition to oxygen, the molecule is also capable of reversibly binding hydrogen as well!

In Summary

  • Pink (Iron - Hemerythrin)
  • Red (Iron - many)
  • Orange (Iridium - chloro-carbonyl-bis(tri phenylphosphine)-iridium)
  • Yellow (Cobalt - Coboglobin, Iridium - chloro-carbonyl-bis(tri phenylphosphine)-iridium)
  • Green (Iron - Chlorocruorin)
  • Blue (Copper - Hemocyanin)
  • Violet (Iron - Hemerythrin)
  • Clear (Cobalt - Coboglobin, Iron - Hemerythrin)
  • Brown (Manganese - Pinnaglobin)

All but two of these (the exceptions are the Cobalt and Iridium based molecules) are known to exist in living organism in the terrestrial biosphere.

  • $\begingroup$ Under erythrocruorin, there is an "and Earthworms." which looks out of place. Was that supposed to go elsewhere? $\endgroup$
    – user
    May 12, 2017 at 9:35
  • $\begingroup$ No. There was a wall of text between "insects" and "earthworms" so I just put the "and Earthworms" after the relevant text mentioning insects. $\endgroup$
    – Jim2B
    Jul 30, 2017 at 16:02

One major issue with blood colors in your scenario is that compounds might look very different when in a dilute solution and when in (near) saturated levels. The best example is blood itself. Considering individual "red" blood cells are golden colored, but give a red impression when seen together in tens of thousands. If you dilute blood sufficiently in clear water, you would see that it is indeed golden of color.

Another big problem is that a lot of metallic compounds have a different color when in anhydrous (dry) form and when forming complexes with water. For example, Copper Sulphate is white when anhydrous and blue when combined with water molecules to form complex CuSO4.5H2O.

Having said that, here are some of your colors.

Tin compounds (+2 and +4 state) are colorless in a solution.

Aluminum compounds are also colorless as solute.

Same for silver.

Reference for above colors

Platinum compounds are mostly brown or red in color. However the oxides (+2 and +4) are black. The peroxide of platinum is golden yellow of color.

Gold oxide (+3) is reddish brown (reference).

Cobalt oxide (+2) is olive green of color. (reference)

Zinc oxide is white while nickle oxide is green (when pure NiO) or black (when the oxygen to nickle ratio is not constant in the sample). (reference)

No metallic oxide that I know of, produces white color when dissolved in water. Metallic complexes might. But metallic complexes vary so immensely in color that it would turn one completely bonkers knowing how many colors a single transition metal ion could have when in a compound form with other metal ions.

  • $\begingroup$ Any idea which of these metals would make decent oxygen exchangers? $\endgroup$
    – user243
    Oct 25, 2015 at 0:59
  • 4
    $\begingroup$ When discussing a metal for practical blood usage, the only factor is not how well the ion can exchange oxygen. One very important issue is, how much of that element is found on the planet and how can it be incorporated in the diet of the creature. Also, how that oxygen transfer compound interacts with the organs of the body cannot be neglected. Yet another factor is the atomic mass of that element and how heavy the blood will make the creature. Having said all that, the only two metals that appear suitable to me for oxygen exchange are Zinc (colorless) and Cobalt (olive green = zombie!!) $\endgroup$ Oct 25, 2015 at 6:30
  • $\begingroup$ Vanadium has a very complex redox chemistry and might make an interesting choice. Several colors , depending on oxidation state. It is already found in some oxidation-related enzymes such as chloroperoxidase. $\endgroup$
    – user11599
    Oct 26, 2015 at 17:40
  • $\begingroup$ V for Vanadium is toxic (not Britney Spears Toxic but biochemically toxic). Specially its oxide is a bad bad choice. Plus, it occurs too rarely to be used as blood. If the creature is non-Carbon based, then it might be a good choice. It has a surprisingly high oxidation state, so even a small amount should be enough to keep the animal functional. $\endgroup$ Oct 27, 2015 at 2:45
  • 1
    $\begingroup$ Lead Oxide ($Pb_3 O_4$), Zinc Oxide ($ZnO$) & Titanium Dioxide ($TiO_2$) form a white powder that is used in paint as a "white pigment". $\endgroup$
    – Jim2B
    May 10, 2016 at 16:51

The colour of a complex organic molecule containing a metal atom is by no means completely determined by that metal. For example, chlorophyll is bright green. The metal it contains is magnesium. Solutions of magnesium salts are usually colourless.

Or you might consider the gamut of colours that a nasty bruise goes through as it heals. These are all generated by Iron, as your body recycles it from blood and other tissues as the damage heals.

Known here on Earth: certain obscure marine worms use Cobalt-based blood. It is (unsurprisingly?) deep blue.

Some of the metals you mention are unlikely to be useful as part of an oxygen-transporting biomolecule. Obviously an alien biochemistry might throw up surprises and the metal or the colour might not necessarily be part of the oxygen transport mechanism. However, to be useful on that front you almost certainly need a metal which exhibits two (or more) oxidation states differing by one. You'll find these in the transition metals on your periodic table. Vanadium through Copper on the first row, and many of the corresponding elements on the two rows below.

The element will need to be reasonably abundant, since an organism will need a fair amount of Iron or whatever it can use in place of Iron. Here on Earth the heavier elements in the second and third rows are relatively rare (and of course you get less atoms per gram). The only one which life has made essential use of, is Molybdenum. (A few bacteria use Tungsten but that's not widespread).

  • $\begingroup$ nigel222 Do you know which protein acts in the marine-worms' cobalt-based blood? I'm guessing that either it's not coboglobin (per Jim2B's answer), or the blood chemistry is sufficiently different that its otherwise-yellow oxygenated state is blue in their case. $\endgroup$ Jul 30, 2018 at 9:22

And don't forget that there are white-blooded fish from Antarctica which don't use metal based oxygen carriers at all (at least in the blood. Some use Myoglobin in the heart muscle).


All the answers above look interesting and with thought behind them, but ph of the solution can change the color and chemical binding of many complex compounds. The reference to the element (probably a metal) being abundant has some credibility. But plants use magnesium for chlorophyl (the structure is very close to hemoglobin) and it is less abundant than iron. It could always be possible for alien biology to use two or three metals together - one that is very common and one that is not - to make a better oxygen carrier. Or some other combination that we can not think of. An interesting side thought - if a copper compound that dissolves in water is added to sodium carbonate, copper carbonate is formed and comes out of solution, but add amonnia and it redissolves. Think about that and alien blood and colors

  • $\begingroup$ Welcome to WorldBuilding, chrales ferris! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ May 19, 2018 at 22:55

Chromium based blood in a +6 oxidation state could possibly appear orange either when it is oxygenated or deoxygenated depending on when it’s in the +6 oxidation state.

  • 3
    $\begingroup$ Welcome to the site Carrmodo. Please note that the Worldbuilding SE is dedicated to providing detailed answers to specific questions a user has while developing his/her fictional world. One-liners are strongly discouraged; answers should provide a solution and explain why that is a good solution to the stated problem. I would suggest you edit your this post to add clarity, otherwise this may be deleted for being inadequate to the purpose of helping the OP resolve his/her problem due to the briefness with which the topic is addressed here. $\endgroup$
    – Frostfyre
    Apr 9, 2019 at 20:49

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