For this mask to be effective, ParticlesSimulate a particle systemRead More needs to have a changing velocity. For these apply some FieldDefines a force-field in 3D spaceRead More over them.
On the example image, all particles are colorized by a Red ColorizeColorize all the vertex of the geometry uniformlyRead More then masked by a Velocity Mask.
Some modifiers, like Colorize Function
Colorize all vertex of the geometry using a and a 3D MaskRead More directly have a mask inside of them.