Light color is measured on a temperature scale referred to as Kelvin (K). Lower Kelvin numbers means the light appears more yellow; higher Kelvin numbers mean the light is more white or blue. Most ENERGY STAR certified bulbs are made to match the color of incandescent bulbs at 2700-3000K. For a warmer light, look for bulbs marked 3500-4100K. For natural light, look for bulbs marked 5000-6500K.