The Nikon Creative Lighting System is built around the incredible capabilities of Nikon Speedlights, and if you've seen flash wizard Joe McNally's advanced lighting videos (if not, check out the links at the bottom of the page), what you've undoubtedly realized is that Joe's flash techniques, methods and experiments are built on his ability to use flash to express his ideas and create a mood with light.
In one of the videos you hear Joe talk about warming up his subject. What he's referring to is the color temperature of the flash unit's light, and it's a key element in flash photography.
A light's color temperature can be thought of as its specific hue, which is measured and expressed in degrees Kelvin. Photographers tend to classify that hue as either warm (toward the orange and yellow range of light) or cool (toward the blue or green range). The lower the color temperature, the warmer the light; the higher the temperature, the cooler the light. As a guideline, consider that the closer you get to daylight, the closer you are to 5000 degrees Kelvin. And the greener a scene—say, a room lit by fluorescent light—the closer you are to 3000 degrees.