First off, great that colors, sharing and changing a single item in a sequence are implemented.

But I would like one more thing. Sync the color of an calendar item if a custom color is set.

I like google's solution. That is a standard default color (1) for a calendar (which can be different for each member and is not synced). And a second color (2), which defaults to (1) but can be changed and is synced. If you look at the item, the first part (most left) is de default color (1), the rest is (2). Items in the past are the same but more washed out.