Why different colour tiles?

Assuming you mean the tiles in Reign of chaos events. In this case the different colours indicate who currently owns (occupies) them. In general the blueish tiles are owned by your own alliance members, and the green ones are yours. White tiles are other alliances from your own province and purple are alliances that are not from your province. When at war any tiles owned by the current enemy are red.

If this is not what you meant please elaborate.