How can I change a particle's colour at runtime?

If you are wanting to use a Particle Parameter, You will need to use Particle Color in your material and then setup an Initial Color Module to use a Vector Parameter (drop the ColorOverLife module). The setup would be the essentially the same in Blueprints without the need to make a MID for the Particle System.

There is a great series by one of Epic’s Senior VFX Artist Tim Elek on our Wiki, I would highly recommend reading through them:

Thank You

Eric Ketchum