How to make a volumetric gradient / fog / haze

Hello!

I am trying to come up with a way to visualize the gravitational forces in an n-body simulation. Basically, I want the player to be able to see where there are stronger gravitational forces.

I was thinking of doing something like a volumetric haze that gets stronger where the gravity is strongest, sort of like colored fog. Something like this image: http://www2.sjs.org/raulston/mvc.10/Topic.2.Lab.2_files/image139.gif except instead of arrows, a smooth fog with gradient coloring nebula effect, eg something like the effect in these two assets: https://www.assetstore.unity3d.com/en/#!/content/11707 and https://www.assetstore.unity3d.com/en/#!/content/40016

I would give those assets a try, but I actually want a gradient and not a solid color, and the "shape" of the fog is not going to really fit a primitive like a cube or sphere very well.

One idea I'm considering is a particle effect where I programmatically place all the particles in a 3d grid and adjust each particle's color individually. I think that will work, but it'll probably be a lot of particles, which is why these fog-like shaders seem like an interesting way to go.

Thanks in advance for any tips or ideas! Alex