The picture below is a great illustration of why you should gel your flash. Do you see the white beam cutting across the middle of the picture? It is pretty obvious that it was made by a different light source than the gold light hitting the rest of the raindrops. The same thing happens when you pop a pure white flash in a room which is lit by normal light bulbs. It really stands out. Adding a simple color-balancing gel (good source here) will help your light play nicely with the light in the rest of the room.
Canon 5D Mark II
102mm - ISO 2500