- and I hope Pr. Gleason feels free to chime in.
The vision of the Trinity is clear from inside the Church looking out; it is muddy from outside the Church looking in. And that exactly mirrors the joyous truth that we come to know the Trinity from inside the Church, singing His praises, receiving His gifts, listening to Him speak to us.
The Holy Father door has a theme of gold, the theme of heaven. First we see the Creator's hand: "I believe in God the Father almighty, Maker of heaven and earth." Then we see the triangle with the tetragrammaton in it: the Hebrew for YHWH and Moses and the burning bush swing to mind: "This is my name for all generations." He is who He is; He causes to be what He causes to be.
The Holy Son doors have a theme of red, blood red. On either side is the Alpha and the Omega (crafted to match that upon St. Paul's altar). They are shining gold as well, confessing the eternity of the Son. The center shows the Lamb, slain from the foundation of the world, waving His banner, victorious in His sacrifice of love, and then giving that self-sacrifice to us in the Holy Eucharist - His true body and blood for the forgiveness of our sins.
The Holy Spirit door has a theme of blue - the color of both sky and water - and we think of the Spirit hovering over the waters at creation and over the Jordan at our Lord's baptism. The Baptism is itself evoked with the sign of the Spirit as dove descending, again with gold work to confess His divinity. The work of the Spirit is confessed with the boat: the Church as the mighty ark of God that brings to salvation, a new world, an unquenchable hope amid the storm and trouble of this life.
From start to finish, the doors proclaim the mystery of the One who has revealed Himself to us: one God in three Persons, to whom be the glory and praise forever and ever! Amen!