Why is my family split into two colours?

Because your family has two clan lines, and the colours keep them apart. Every person belongs to a clan-identity side derived from their clan key or birth surname, and the tint follows them everywhere — the tree, the calendar, the timeline — so you always know whose side you are looking at.

The colours are stable for you: the same side wears the same colour on every screen. Your two sides are simply the two families a marriage joined.