I don't think that there is any order of colours for Angels. In recent centuries it seems usual that they are portrayed in white. Before that, they were portrayed in red because that was the colour of power.
This Icon dates to about 1390. It is considered to be a kind of window into heaven. Icons focus meditation and welcome the presence of God into one's life. The Icon is composed of three identical Angels. The colours of their robes reveal which person they represent. They are sitting around a banqueting table, the Altar. The Angel in the middle is blessing the chalice and bread and is clothed in purple, the sign of nobility, mourning, penance and sacrifice. He is Jesus. The Angel to the right wears green, the symbol of new life and renewal of the Church. He is the Holy Spirit. The Angel to the left is in Gold. He is the Father, creator of the world in whom all things exist. The positions of the Angels form a Chalice with the Angel of Christ in the middle. Above the Angels is a tree of life in full bloom, a stone to the right represents the rock of faith and the building to the left is the temple of God. There is an open space in the front of the table. This is for us, the viewer, as we are invited to join in this banquet of God.