Help Solver.OnParticleCollision not work + mix 3 color

[hungseparate]

"use the x,y,z diffusion channels to store color r,g,b"

As i understand, i need to loop through all particles, get it rgb value, then set it back to userdata ?

then other particle will receive this userdata, calculate new color ?

I don't really understand this userdata, it store current particle's value ? or store value of particle next to it ?