The dcvColor member of the D3DLIGHT2 structure specifies a D3DCOLORVALUE structure. The colors defined by this structure are RGBA values that generally range from zero to one, with zero being black. Although you will usually want the light color to fall within this range, you can use values outside the range for special effects. For example, you could create a strong light that washes out a scene by setting the color to large values. You could also set the color to negative values to create a dark light, which actually removes light from a scene. Dark lights are useful for forcing dramatic shadows in scenes and other special effects.
When you use the ramp (monochromatic) lighting mode, the ambient light is built into the ramp, so you can't make your scene any darker than the current ambient light level. Also, remember that colored lights in RGB mode are converted into a gray-scale shade in ramp mode; a red light that looks good in RGB mode will be a dim white light in ramp mode.