Direct3D Immediate Mode describes color in terms the four components (red, green, blue, and alpha) that combine to make a final color. The D3DCOLORVALUE structure is defined to contain values for each component. Each member is a floating point value that typically ranges from 0.0 to 1.0, inclusive. Although both lights and materials use the same structure to describe color, the values within the structure are used a little differently by each.
Color values for light objects represent the amount of a particular light component it emits. Lights don't use an alpha component, so you only need to think about the red, green, and blue components of the color. You can visualize the three components as the red, green, and blue lenses on a projection television. Each lens might be off (a 0.0 value in the appropriate D3DCOLORVALUE structure member), it might be as bright as possible (a 1.0 value), or some level in between. The colors coming from each lens combine to make the light's final color. A combination like R: 1.0, G: 1.0, B: 1.0 creates a white light, where R: 0.0, G: 0.0, B: 0.0 results in a light that doesn't emit light at all. You can make a light that emits only one component, resulting in a purely red, green, or blue light, or, the light could use combinations to emit colors like yellow or purple. You can even set negative values color component values to create a "dark light" that actually removes light from a scene. Or, you might set the components to some value larger than 1.0 to create an extremely bright light.
With materials, on the other hand, color values represent how much of a given light component is reflected by a surface that uses that material. A material whose color components are R: 1.0, G: 1.0, B: 1.0, A: 1.0 will reflect all the light that comes its way. Likewise, a material with R: 0.0, G: 1.0, B: 0.0, A: 1.0 will reflect all of the green light that is directed at it. Materials have multiple reflectance values to create various types of effects; for more information, see Material Properties.
Color values for ambient light are different than those used for light objects and materials. For more information, see Direct Light vs. Ambient Light.