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.
But you needn't think about the actual degrees Kelvin of a light source. When Joe said he wanted to warm up his subject, he wasn't concerned with the specific temperature of the light; rather, he wanted his flash to cast a warm, slightly orange-hued light.
With a Nikon SB-900 Speedlight, it's incredibly easy to achieve that look. Supplied with each SB-900 is a set of color filters—Joe refers to them in the video as "smart filters;" they are also sometimes referred to as "smart gels."
Slip a filter into the supplied color filter holder, attach the holder to the flash head and you're ready to influence the color of the light.
The incandescent (tungsten) smart filters warm up the scene or subject; fluorescent filters cool things down. Both are supplied in two densities. Equally important, the filters will balance the scene's ambient light with the light of the flash.