Revelation 4:2-3

2 And immediately I was in the spirit: and, behold, a throne was set in heaven, and one sat on the throne.

3 And he that sat was to look upon like a jasper and a sardine stone: and there was a rainbow round about the throne, in sight like unto an emerald.

John was in his spiritual body and was taken immediately up to the third heaven. I think the apostle Paul also saw some of these things when he was caught up to the third heaven. He said he did not know if he went in his body or departed from his physical body. Paul could also write about the mystery of the rapture of the Church in his letters because he also had a foretaste of it when he was caught up to the third heaven.

2 Corinthians 12:2 I knew a man in Christ above fourteen years ago, (whether in the body, I cannot tell; or whether out of the body, I cannot tell: God knoweth;) such an one caught up to the third heaven.

John tells us about the throne he saw in heaven and the description of the one who sat on the throne. The text in this passage should say  "and there sat on the throne"  not "one sat on the throne"; the word "one" is not in the Greek text. John and the Church are in the spirit looking upon the throne of God. The Father was certainly there with Jesus on this throne but since scripture tells us that no man has seen the Father (except the Son) and that no man could see the Father and live, John could only have seen the Son of God.

John 1:18 No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him.

What is actually seen here by John is Jesus sitting on the Father's throne. Jesus is the express image of the Father.

Hebrews 1:3 Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high;

The description that John gives also indicates that he sees Jesus. The description is that of a high priest; the jasper and sardine are the first and last stones in the breastplate of the high priest (Exodus 28:17-21). The Son of God from eternity past to eternity future is reigning upon the throne of the Father. The rainbow around the throne is a refraction of the full spectrum of light coming from God. The emerald is the stone of the tribe of Judah. The rainbow that had the appearance of an emerald did so because the light from God was shining through Jesus who came from the tribe of Judah.