We know it was not the light emanating from God, Himself, for that light has always existed; "This then is the message which we have heard of Him, and declare unto you, that God is light, and in Him is no darkness at all" (1 Jn. 1:5).
A Pantheist would quote Albert Einstein's Theory of Relativity (Energy = Mass times the Speed of Light squared), as proof all of creation is God, and God is all of creation. According to Merriam-Webster's Dictionary, pantheism is the (false) "doctrine that the universe is God and, conversely, that there is no god apart from the substance, forces, and laws manifested in the Universe." Einstein would have had to have said that God, being light, simply "slowed down" to create our universe. In order for Einstein's theory to define Light, the formula would be Light = Energy divided by Mass times the Speed of Light. Do we really think all of creation is "God in slow motion?"
We know it was not the light from the Sun, the Moon, and the Stars also (also - hmmm, so is our Sun really just another star?), because they were not created until the fourth day of Creation (Gen. 1:14-19). So, in reality, we do not know what the light of Genesis 1:3 was.
From Wikipedia, the free encyclopedia: In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks. Newton divided the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet. I am not sure how this relates to Revelation 4:5, which says, "And out of the throne proceeded lightnings and thunderings and voices: and there were seven lamps of fire burning before the throne, which are the seven Spirits of God." Just something to ponder. LOL!
God shares just enough "light" to keep His children humble!