Use a map (can be a Shared Element
too). To color each vertex based on it's UV.
To use this modifier over a particle set, either emit particles from a geometry or use a UV Mapping
As you'll see on the example below, that is what mean "per vertex color".
On a low subdivided plane (10x10) you'll get something like that :
On a medium subdivided plane (64x64), you'll get something like that :
This modifier is so particularly effective over a particles system. You can use a Video Input
as map to colorize your particles with what comes out of your webcam.
You can also try to combine it with Layer Intensity
or Normal Map Layer
+ Optical Flow
to a maximum funky fresh effect to the over top.
Here is a tutorial example using this modifier :