The Kelvin scale describes the color of a light source when compared to the color a theoretical "blackbody" radiates when heated at the same temperature. Think of a piece of steel that changes color as it is heated, then measuring the color of the light being given off.
At 0° Kelvin (equivalent to -273° Celsius), the theoretical blackbody emits no light. As the blackbody warms, it begins emitting red light. As the blackbody increases in temperature, light wavelengths become more yellow, then green, blue, and finally violet.
For example, sunlight at noon, with a Kelvin rating of 5500°K, is typically referred to as full spectrum because it contains a blend of all colors throughout the spectrum. Bulbs with a lower K rating (and lower color temperature) emit a warmer, reddish light, while bulbs with a higher K rating (and higher color temperature) give off a cooler, crisper blue light.
To best duplicate natural lighting conditions under which your aquarium inhabitants will thrive, pay close attention to each light bulb's Kelvin (K) rating, which measures the bulb's light spectrum or "temperature." Select lighting with the K rating that will give your inhabitants the light they need to thrive and grow.
The Right K Rating for Your Aquarium