I bought Samsung s8 few days back and I was trying to change my existing calendar event color and I tried everything but no luck.I can change or select my new event color but not for existing.
Where are the events stored/coming from? If the calendar they're saved to doesn't allow edits (or doesn't support custom colors) you won't be able to change them. If you create an event into a calendar that supports it (like your Google account, for instance), then when you open the event details you should see the color next to the event title. Tapping that would prompt you to change the color if you want.