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Based on | difSource – Specifies the source channel that will be used to determine the smoke color. By default, a uniform simple color set by the Constant Color option is used. This could also be a texture map, or read from the cache files - in that case the corresponding grid channel must be present in the cache files and remapped to the internal Volume Grid channels using the 3rd Party Channels Mapping dialog in the Input roll-out.

Disabled - the smoke color will not be rendered.
Temperature - the smoke color will be based on the values of the Temperature cache channel and can be remapped using the color gradient.
Smoke - the smoke color will be based on the values of the Smoke cache channel and can be remapped using the color gradient.
Speed - the smoke color will be based on the values of the Speed cache channel and can be remapped using the color gradient. See the Smoke Color by Speed example below.
Texture - the smoke color will be based on the values of a texture map plugged into the Texture slot.
Constant Color - the smoke color will be the same for the entire volume and will be taken from the Constant Color box.
RGB - the smoke will be based on the values of the RGB channel.
Fuel - the smoke will be based on the values of the Fuel channel and can be remapped using the color gradient.

Texture | difTexture – If Based on is set to Texture, this slot specifies which texture to use.

Modulate | difModulate – When Based on is not set to Texture, the color is multiplied by the texture in the Texture slot.

Reset to Defaults  – Resets the settings to their default values.

Constant color | difSimpleColor – If Based on is set to Constant color, this color is used. 

Scattering | difScattering – Controls how the light rays are scattered inside the volume.

Ray-traced (GI only) – Enables physically accurate scattering of light rays. This mode produces the most realistic results but it's the slowest to render. It requires V-Ray with enabled Global Illumination, otherwise, the rendered result would be the same as if the option is Disabled. The Master Multiplier does not affect the rendering in this mode.
Disabled – Disables scattering. The Master Multiplier value can be used to correct the brightness because without light scattering the volume would generally render darker.
Approximate – Uses an approximate formula which is faster than Ray-Traced scattering and produces good-looking results. Brighter areas of the volume would transfer light farther than dark areas. This option is not supported by the Volumetric Geometry render mode in the Rendering tab.
Approximate+Shadows – Same as Approximate, but also affects the strength of shadows over the scene geometry. Approximate+Shadows is not supported by the Volumetric Geometry render mode in the Rendering tab.

Move the slider to see the example renders.