Christ sits on a throne because he is king; Christ on the throne represents Wisdom: Wisdom is king. God creates through his wisdom, through his Son. In the icons of the Pantocrator, although creation is attributed to the Father, the Father is never represented, the Father does not have a human face, but Christ, it is always through the Son that God creates: "Whoever has seen me has seen the Father" (Gospel according to Saint John ch.14, v.9).
Christ is in blue, the colour of the high, the colour of wisdom, which is the gift of tasting God, it is contemplation, communion, meditation, union. To taste God is to live of his life.
Christ is also dressed in purple, the colour of royalty.
He is seated, because he is well established, he is king for ever. His feet are placed on a pedestal; kings never put their feet on the ground. Jesus is elevated above the lower plane.
Christ is the Word, which is why he holds the book in his hands.
Sister Marie-Paul