You can use the Particle Simulation Settings to define your own variables that can be automatically linked to the particles and filled with your own values. These variables and their values can then be evaluated and also rewritten using various emitters, conditions and modifiers.
All user-defined properties for the particles are listed on this settings page, allowing the desired start values to be transferred directly to these user properties for the new particles created at the emitter.

If there are no custom particle properties in the simulation scene, this dialog page is not displayed.