The fullness of Col. 1:10 cannot be a fullness of grace, though, can it?
Do you know what it means to be full of grace? That means your will is perfectly conformed to the will of God and there is no sin in you.
It means you are full of God's gift. That's why Colossians says believers have fullness in Christ.
They are blameless in Him as is said many times in the New Testament.