Kelvin is the color of the light. The old light bulbs that most people grew up with were almost exclusively 2700K. That is the color of most standard 40 to 150 watt bulbs you would purchase at the grocery or hardware store. They have a slightly orange warm glow. Next is 3000K which is a little more white and then it moves up to 4000K which is almost like an operating room. Very white.