Think of the 6500K "Daylight" CFL as your spring and summertime light. This is when the sun is highest in the sky and gives off more Blue light spectrum than red. Once the sun goes into a fall and wintertime place in the sky, there is only 12 hours of light. Also, because the sun is lower in the sky, it gives off more red light spectrum (2700K). People use the different lights to emulate how the sun gives energy during the seasons.