Choosing the color spectrum of a fluorescent tube is a personal preference item. The warm white and cool white bulbs are far more yellow than sunlight but give a warmth to the things they illuminate. If you like brighter and starker colors, a tube with a rating over 6500K will tend to make colors brilliant but the overall effect is almost sterile in my opinion. Anything of 10000K or more is simply bluish in color and not going to bring out any natural colors in a tropical tank. A tube with a rating around 5500K is replicating sunlight as far as its dominant wavelengths but may or may not bring out what you are looking for.
