Can someone explain to me why and when some words use the "ω" and others "ο"? Is there a rule for that?
The Greek language alphabet letters, όμικρον O o and ωμέγα Ω ω, sharing the same English language alphabet letter O o, do not share the phonetic pronunciation.
Xylophone ξυλόφωνο is an example word to demonstrate the greek small and large "O" sound / pronunciation difference.
όμικρον · ó-mi-cron · | · μικρο · mikró · small micro · | όμικρον = small ό | ξυλόφωνο · xylófono
ωμέγα · oméga · | · μέγ még max · | · μέγα · méga · | · ω-μέ-γα · o-mé-ga = large ω · | ξυλόφωνο · xylófono
The "O" sound in ξυλό xyló [ small O ]
The "O" sound in φων fon [ large O ]
And a lot of words which have something to do with colour start with the chroma- prefix, e.g. chromatography
If you have a tabletI you can go into settings and click on language and input you should be able to activate the greek keyboard. Then when tou are typing click on the button that looks like a circle with line.