Darkness and Light:
Paul pointed out that Christians are rescued from the dominion of darkness (ignorance, falsehood, and sin, and in the case of the Colossians, heresy) and brought into the Kingdom of the Son, or the Kingdom of Light.
Why does he remind them where they came from?
Because he did not want them to go back through the false teachings.
The inheritance of the saints in light:
To be brought into the Kingdom of the Son, under the sovereign rule of the Lord Christ over human hearts.
The contrast of the governing bodies:
It indicates the contrasting spiritual realms. The one is dark, malignant, and controlling. The other is filled with light, hope, love, redemption, and forgiveness.