What's the difference between ερυθρός and κόκκινος?
Is it like the difference between γαλάζιος and μπλε, i.e. different shades? Or is it something else? Or are they perfect syonyms (if such a beast exists)?
I think it's a difference of register, like ασπρός and λευκός.
Ασπρός and κόκκινος are the words commonly used in everyday language. By contrast ερυθρός and λευκός are formal and literary, with their everyday use confined to some fixed phrases (e.g. λευκό κρασί, the Λευκός Πύργος in Thessaloniki, Ερυθρός Σταυρός (the Red Cross)).
Yes, exactly. Aside from what you've mentioned, the only place I've ever come across ερυθρός has been on wine bottles :-) Λευκός is commonly still found in place names (as in Λευκά Όρη), plus it made it into English (leukaemia). But that's about it - they're not words that are ever used in everyday speech.
And white and red blood cells in biological terminology are leukocytes and erythrocytes :)
Yes no one really uses the word ερυθρός anymore. The only usage of the word I can think of is for football team names or nicknames, such as ερυθρόλευκοι (red and white, for the football team Olympiacos, probably because this nickname became popular in the 1950s) or Ερυθρός Αστέρας (for the Serbian football team Red Star, probably because the team's name translation was done again in the '50s or '60s).
What is the (non scientific) Greek expression for the red blood cells (erythrocytes) ?
I don't think that there is a scientific and a non scientific word for this, as the subject is scientific in itself. Erythrocytes are ερυθρά αιμοσφαίρια or ερυθροκύτταρα.
We usually say : λευκά δόντια= white teeth, λευκό χαρτί=white paper, λευκή σημαία=white flag, λευκά ρούχα = white clothes, λευκό δέρμα = white skin..
This is not about Greek per se, but in English erythrism refers specifically to an animal that has red hair or fur.
Ερυθρός Σταυρός, ερυθρά αιμοσφαιρια. Ερυθρός is an old word, from ancient Greek. Κόκκινος is a daily word.