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.
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).
This is not about Greek per se, but in English erythrism refers specifically to an animal that has red hair or fur.
Native Greek speaker here. We rarely ever use ερυθρός, it's considered kind of "too official" or archaic, the only cases I can think of is Ερυθρός Σταυρός, the Red Cross organization, and ερυθρά αιμοσφαίρια, red blood cells. Ερυθρός technically means "deep red", so you can also describe blood or deeper-colored things. Καλή ερώτηση!