On the example Image, all particles are colorized by a Red Colorize Colorize all the vertex of the geometry uniformly Read More then masked may an Age Mask.
This can also be achieved with a lot more options by using a Function Enables to use any other type of 3D mask with a Function (for advanced users) Read More set as Age.
This mask allow to use Field Defines a force-field in 3D space Read More or 3D Modifier Modify a 3D geometry Read More only on a defined period of a Particles Simulate a particle system Read More lifespan. If you want for instance for gravity to affect only dying particles, just set a function mask as Age over a Gravity Apply a directional force to particles Read More and make it work only on the end of the lifespan. Here is an example: