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This roll-out controls the Diffuse color of the Volumetric Shader. The Smoke Color can be a uniform color, a mixture of colors dragged by the simulation (RGB), a function of any physical channel, or sampled from a texture. It is also affected by lights and shadows cast by objects in the scene.

In the volume shader, there are two types of content - diffuse and emissive. For simplicity we call the diffuse content 'Smoke' and the emissive content 'Fire', though the volume shader is flexible enough that you can render a simulation's fire as smoke and a simulation's smoke as fire.

Diffuse needs external light in order to become visible, and also casts shadows. Emissive on the other hand is visible even without being lit by lights, ignores their illumination and does not cast shadows on itself. In addition, Phoenix has different tools that help you gain more control over how fire illuminates the smoke and the scene - see the Create Fire Lights section.

Both smoke and fire have their own color and alpha (alpha is a synonym for opacity, and transparency is the opposite of opacity).

Fire color and alpha, smoke color and smoke alpha can each be mapped to a physical grid channel, coming from the simulation. Color gradients are used to remap a physical channel to render color, and the diagrams (also called ramps or curves) are used to remap a physical channel to render opacity or intensity.

Also, each of them can be mapped to a texture, or to a grid channel multiplied by a texture. Textures have infinite resolution and can increase the detail above the resolution of the grid; By default textures are static in space, but using TexUVW they can move together with the fluid.

When there are both diffuse and emissive (smoke and fire) in the same voxel, there are 3 ways to determine the resulting color and alpha in that voxel - see the Fire Opacity Mode option.

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

Scattering | noscatter – 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.
Approximate+Shadows – Same as Approximate, but also affects the strength of shadows over the scene geometry. This option is not supported by the Volumetric Geometry render mode.

1. This option refers to the internal Phoenix FD Light Cache, which is not related to the V-Ray Light Cache.
2. When using V-Ray Progressive Rendering, this option might slow down rendering startup or the overall render speed. 
3. This feature might produce artifacts when the smoke is very dense.
4. This feature might produce artifacts when objects cast shadows through the smoke.
5. Rendering multiple copies or instances of the simulator node with this option turned on might lead to slow-downs.
6. This option will consume additional memory, so beware of high RAM usage when rendering very large cache files or a large number of copied or instanced volumes.
7. When rendering in Volumetric Geometry mode, for any of these Render Elements: [ Shadows ], [ Raw Shadows ], [ Global Illumination ], [ Raw Global Illumination ] , the Volume Light Cache will require additional memory to produce the correct result. Please be aware that these Render Elements (and most other V-Ray Render Elements except for [ Atmosphere ] and [ Self-Illumination ] ) do not work in Volumetric mode.
8. This option is ignored when using V-Ray GPU. It is a CPU-only feature that helps speed up Bucket rendering.

 You can use the following controls in the color gradient:

Double click – Creates a new point or changes an existing one.
Left button drag over a point – Moves the point. If several points were selected beforehand, they will move the same amount.
Left button drag over several points – Selects several points.
Middle button drag over the background – Drags the visible area.
Mouse wheel – Zooms in/out.
Right click – Displays a drop-down menu where you can add a point, edit or delete a selected point, and fit the entire gradient into the view. If multiple points are selected, they can be edited simultaneously.

Example: Smoke Color Based on Speed