Here is some krakatoa coloring techniques.
3dsmax 2012 scene with PFlow setup, as well PRT Loader with already pre-cached one frame (220MB). All MagmaScripts with exact screenshots included. Magma files can be download separately under each thread or from dropbox.
* not for commercial use.
Position | Magma flow
Atmospherical gizmo object is used as placement area for blending two colors.
After some tweaks, there is possibility to use more than one gizmo.
Or even custom geometries.
Or color by nearest particles from another particles container.
Density | Magma flow
Density can be modified individually, even in another magma flow. Here is example how to set noise map as particle opacity.
Gradient | Magma flow
Usually is better to use 3dsmax maps to generate color data, as example gradient ramp as particles velocity mark.
Or even better – leave entire color info to maps/other sources and modify only particles texture coordinates. Here is the same results like previous, but completely different approach.
One of the most wonderful Krakatoa tricks is that same particles coloring approach can be used for render elements (as CustomData).
Here is example how one more color was added to the final image by extracting and masking one velocity direction from Velocity Pass. With custom elements any maks can be created.