Evolution of the Greek Language

The Greek language is an offspring, one of several, of the large family of languages modern science calls the Indo-European group. Looking at the interrelationships of these languages, one usually pays more attention to sound and meaning than to the alphabet used by that language.
Considering the written word, Sanskrit, Greek and Swedish are miles apart; but certain words, sounds and meanings are closely interrelated or almost identical.

Matir (Doric Greek), Mater (Latin), Matr (Sanskrit), Miter (Attic Greek), Mitera (modern Greek), Moder (Swedish), Moeder (Dutch), Mother (English), Mutter (German) - there can be no doubt that we're dealing with one mother language; and in the absence of a real name, we call it Indo-European.
And because that language is at the root, it also comes at the bottom of the following chart.

The development of Greek, in itself, has followed a similar pattern. At first there were clear, but not terrible, differences with five separate streams of Greek coexisting for hundreds of years. However, when Alexander (356 - 323 BCE) forged the great Greek Empire and Greek became the dominant language from Rome to Syria, from Greece to Egypt, Anatolia and Judea - the Attic dialect turned into a language common to many peoples and countries. During this process (indicated by the three blue arrows from Attic to Koine Glossa, it did absorb certain verbs and phrases from the other Greek dialects - indicated here by the one arrow coming from Doric).

That way, the classical Greek language came to exist which was then used to translate the Hebrew Old Testament and to write the Christian New Testament. It is from that classical Greek, throughout the following centuries, that one arrived at the (modern) Greek we're all trying to learn and speak today.

Well, I found all of this out while searching for the reason that the Cretan dialect is different from the Greek spoken on the mainland. And there it is: Cretan is a relative of Doric - while modern Greek is more deeply rooted in Attic. So when a Cretan pronounces the color RED as kocheenno, it's a reminiscence of old Doric - and when your computer based language course pronounces RED as kokkeenno, then they're using modern Greek as spoken on the mainland.