Change Particle attributes when inside a specific area Do you know a way to change some attributes of particles, when they enter a specific area (e.g. change color/material, make them slower)?!

I also want the particles that enter a specific area to stay inside it (e.g. don't fall through it's bottom face).

I am trying to let the particles fill out a specific mesh but don't know where to start.

This is how far I am:


The "0" and "1" are my particles (always facing the camera).
I want them to get slower and change their material inside the weird shaped Mesh (and the object should be invisible in the final render). 

For now I just gave it a collision, I don't know where to start wwith this task. Thought about adding forces but ... meh... 

Blend file:http://www.megafileupload.com/596c/particles01.blend

I want to change the color/material within Cycles renderer in the end.
But more important are alterations to the particle speed, so that you can guess the shape, if there are "enough particles inside the invisible object".
Keyed Particles is one approach

Note Blender Render, Cycles not considered



The green sphere holds the plan for particles moving from the yellow cube to the red tall box.
This is a starting point for further research.

You stated 
[I don't know where to start wwith this task]