What is the significance of a rainbow?
Malbim: The Nogah that he perceived was light reflected from the supreme light - hidden light on the Merkavah. It seemed like a rainbow on a rainy day. Light that is intrinsically one, it appears divided on the cloud in seven colors. So the special light in its source appears divided when it is pertains to conduct of living beings in seven known shades. Light of the Sefiros breaks into seven lights. And just like light of a rainbow has no existence in the cloud itself, just it appears on it via light of the sun that is reflected in the clouds and appears like a rainbow. A rainbow is not seen if the light passes without obstruction. So Divine (intellect) light is not associated with any color or image. It comes on the Navi's Nefesh in a thick cloud, i.e. imagination. It returned to be seen in these images that he saw.
Why did he fall on his face?
Rashi: He bowed.
Radak: He was astounded at the vision.
Malbim: His senses and powers were Batel, and the power of imagination remained to see and influence on the intellect. From this side, the Nevi'im saw visions, like it says in Moreh ha'Nevuchim.
What did the voice say?
Rashi: It is what it says in the coming verse.
Radak: While he was on his face, he heard an angel speak with him.
Malbim: It was a voice of Nevu'ah.
