A few questions about Particle System II / World?

In case you're still looking for answers using Particle Systems II.

To prevent particles from floating away, set the particle velocity to 0.
To change color over time, it's much easier to simply duplicate your particle layer, and tint each layer with the appropriate colors. Now create a mask that intersects the exact point where the color should change.

Particle Systems II is a very simple particle generator, but will do exactly what you want.

Particles are assigned their over life attreibutes at their birth. This isn't a thing you can keyframe on and off based on absolute time. That's what the various graphs and gradients do once you enable them. Similarly, there's a "orient to motion direction" or whatever it is called in the particle properties, but it's only available for cloudlets and textured particles, not the basic point particles. Otherwise there's a million ways to skin this and in fact most of the time one would rely on the secondray particles emiited from the main particle group rather than the primary particle group. That could also solve your coloring issue: You define a collision plane set to Event representing your membrane and from that you emit your other particles/ switch the particle group. Your emission could be anything from a simple line mask to a grid emitter with just a single row/ column. Even a basic directional emitter would work when you tweak the size suitably and reign in the particle forces.

Mylenium

Unfortunately there are barely any tutorials even mentioning Playground. Ever since Particular came out in 2002 people didn't bother any longer and just switched right away and even before that it was not that popular. If you have specific questions it might be best to post screenshots of your artwork and maps and I may be able to nudge you in the right direction, even if I haven't used the effect in ages myself.

Mylenium

You cannot ifluence anything with scripts or code. The old rule applies: Expressions have no magic. They are only a different way of controlling properties that exist, anyway. The effects would have to have extra "per particle rules" inputs, which they don't. They are just the most basic particle systems. If you need more control, you have to invest in advanced plug-ins like Trapcode Particular or Stardust. They do offer "sticky" flow along path options. Similarly, the coloring options using the "over life" option and changing the start and end colors are limited, so chances are for that you have to move on to other tools as well. For a simple side view it could be as trivial as using two layers and masking the filter membrane or clipping it with a linear wipe and then coloring the particles differently e.gt. with a Colorama or Tritone effect. Either way, you have to approach this quite differently. Trying to do such stuff in a single sweep rarely ever works, even in advanced particle systems or 3D programs.

Mylenium