Revelation is a prophetic vision (a dream if you will) of what John saw, and his interpretation of that vision. It heavily relies on symbolism and metaphors, where certain images are used to represent something else. Yes, there are some passages that are purely literal, such as when God says "I am the Alpha and the Omega, the Beginning and the End". We know that this is literal because elsewhere in the bible it describes God as being eternal. But what is literal and what is allegory in Revelation can easily be determined just by comparing the scriptures written in the book of Revelation to other scripture. If/when a passage in Revelation seems to contradict scripture elsewhere, then the reader must be misinterpreting it by taking it in a literal sense when they shouldn't be. The Lamb being the light is clearly a metaphor. For one thing, Jesus is not a literal Lamb, he is a person! To take it literally would not make any logical sense. So right there we have a metaphor within a metaphor. The logical reading of the passage would be to conclude that Jesus being the "light" is also a metaphor, and not literal visible "light".
I agree with all of that. This is a logical and consistent way of understanding scripture based on the context given within the Book of Revelation and outside of it.
But that's not what you suggested initially when you referred to God being the "light" without the sun in Genesis 1. Correct me if I'm wrong, but it sounded like you were suggesting that the light that God created in the beginning was literally coming from himself and not the Sun.