For this mask to be effective,
Simulate a particle system
needs to have a changing velocity. For these apply some
Defines a force-field in 3D space
On the example image, all particles are colorized by a Red
Colorize all the vertex of the geometry uniformly
then masked by a Velocity Mask.
Some modifiers, like
Colorize all vertex of the geometry using a and a 3D Mask
directly have a mask inside of them.