Question

Is human blood yellow?

Answer 1

Human blood consists of haemoglobin protein, which helps to bind oxygen with it and this pigment describes the colour of blood. Not all blood proteins bind with oxygen, the binding of blood protein and its colour depends upon the habitat of organisms.

Complete answer:
The colour of blood is dependent on the protein pigment present in it. Human blood mainly consists of a fluid matrix, plasma and blood corpuscles. In blood corpuscles, RBC consists of haemoglobin protein pigment with an iron element in it. This haemoglobin pigment binds with oxygen and makes oxyhaemoglobin which helps in the transportation of oxygen in the blood. After binding of oxygen in haemoglobin the dark red colour of haemoglobin gets converted into bright red colour and due to this human blood appears bright red.

Additional information:
The yellow colour of blood appears in the species of sea cucumber in which pigment vanabin is present. The vanadium element in vanabin makes the colour of blood yellow. The vanabin element does not get bound with oxygen, it only produces chemicals for self-defence. Therefore these organisms also consist of haemocyanin for the transportation of oxygen but haemocyanin is not responsible for the colouration of blood.
In some arthropods like the spider, hemocyanin pigment consists of the copper element which produces blue colour to the blood and it also helps in oxygen binding and its transportation.

Note:
Some living organisms have colourless blood, like Antarctic ice-fish. It does not consist of blood corpuscles, haemoglobin pigment and not any oxygen-carrying pigments. Therefore the blood of these organisms carries $90\%$ less oxygen as compared with other organisms.