This is where the metaphors all begin to collide and combine.
This revives the metaphors from the first verse: light/color, darkness & water; and ties them into a new metaphor through allusion to the story of Noah.
Remember that light and color represent a positive and strong natured relationship. Darkness represents a negative and week relationship. Water represents something that in little bits can be good in our relationship but can also be dangerous and destructive in large amounts.
It takes light to make a rainbow, so it is impossible to find one in the dark, yet I still look. Even If I reach the end of a rainbow, I will only find a pot of fool’s gold instead of real gold.
Color is just how our mind interprets wavelengths and it is not actually tangible or acquirable.
Basically, I am running impossible errands with no possible reward, yet I can’t help searching.
This ties in with Noah because he had to build an ark (arc) which he’d hoped would help him survive the imminent doom brought on by massive amounts of water, only to land in a cleansed world. He sailed for forty days and forty nights and was finally rewarded with a renewed land and a rainbow that symbolized how a negative situation can bring an outcome more positive than before. I, like Noah, will search with little hope in order to find my ultimate reward.
Remember what happened last time I was on a boat?
This likeness to Noah brings on an even bigger metaphor that encompasses the entire song.