The idea of color temperature is to give you an idea of how hot an object would need to be to radiate light of a similar spectrum. Since the surface of the sun is variously cited as being 5500K to 6000K, a lamp with that spectrum is likely to give a color of light that resembles noon daylight. A 3500K lamp is like most so called cool white bulbs, it is quite a bit yellower in color than a 6500K. A 10000K bulb is quite a bit bluer in color than a 6500K bulb. If you are not growing plants, the bulb color choice is a matter of your personal preference. If you are growing plants, it is suggested that you stick with bulbs between 5500K and 10000K because the plants can make better use of light in the spectrum that comes from those bulbs. At least that is what plant people will tell you. Bulbs are also made much bluer than a 10000K bulb and are called actinic. Those are in a color similar to what you might find after sunlight has filtered through many meters of seawater. They are usually used by people with saltwater tanks to promote better health in their corals. The only place they have in freshwater is if you like the color and do not have any plants.
