Prior to digital colourisation, much of comic book colouring and printing was done with a limited set of pigments and inks. This progressed and diversified over the years though one spot of discussion, if not contention, is how black hair and objects are often depicted with blue highlights.
According to one blogger, sometimes those characters in dark blue costumes are supposed to be clad in black but due to inking and colouring limitations it can’t entirely be pitch black. Spider-Man is supposed to dress in red and black. The blue colouring is an immediate but later development.
In the case with black-haired characters, at the time they couldn’t have grey highlights but they also couldn’t have brown highlights either or otherwise they’d have dark brown hair. So the colour blue was the next best thing.
This however led to certain interpretations and contention over whether the character actually has blue hair or not. It can be said that in countries where black hair is rare, dyed blue-black hair takes its place. Indigo is the go-for blue dye that can make hair a dark blue or purple.
Interestingly, blue hair is already possible in some animals like cats and dogs but it’s genetically a diluted black and the colour is a dull dark blue-grey. Sometimes it looks bluish and sometimes it looks like a dark brownish grey.
Then we get to how red hair is drawn. In comics and cartoons, blond hair is rendered yellow. White hair is either pure white, greyish or yellowish. Black hair is either black like it is in real life or bluish. Red hair is red but the way cartoonists interpret it is either a bright red or orange.
Since red-haired humans are rare, it wouldn’t be surprising that most people have a hard time pinning a good red hair colour down without making it bright red or orange. The best that they should do is to look to a rufous ape called the orangutan. Sometimes human red hair can be as red as that of an orangutan.
White hair is another matter as it’s sometimes shown to be either pure white, greyish, yellowish or bluish. Yellowish white hair is much more possible in real life as white hair looks blond when set against the whiteness of some objects. Greyish white hair would just be white mixed in with black or brown.
Some linguistic notes:
In some languages like Irish or Igbo, though depending on the dialect for the latter, there is an immense overlap between the colours blue and black. One of the Igbo words for blue is amaloji and the Igbo word for black is oji. In Irish, gorm is usually blue but can refer to what Anglophones would call black skin.
So far in Irish, Polish and Portuguese there is a separate word for red hair (or dull red things) and bright red. In Portuguese, it’s ruivo if male and ruiva if female. The word for bright red and scarlet is vermelho/vermelha. In Polish, rudy/rude is what you use to mean red hair while czerwony is red and scarlet.
Similarly in Irish, rua is what you use to mean red hair and dull red things while bright red objects are dearg. In that same language, glas is used to refer to the greyness of sheep and horses as well as the greenness of vegetation but focal green is uaine.