The Kelvin rating refers to the color value assigned to the the bulb. The color of an LED that has not been corrected is normally a bluish color or has a high Kelvin value. A cool white LED approximates the color of a typical halogen lamp so the Kelvin value is lower. An LED with a yellowish cast has an even lower Kelvin value. An example of a Kelvin rating will look like this. ( 5500K)