One very useful feature of the UE4 Material system is the Black Body Material Expression node, which can be found in the Material Editor. Beware, we now have to dive into SCIENCE!
Black-body is a type of radiation that specific types of objects (or bodies) emit at a certain temperature. One of the visible characteristics of this radiation is the spectrum of color of the light that is emitted.
In simple terms, you give the node a real-world temperature in Kelvin, and you get a real-world color corresponding to that temperature. This solves that age-old problem of "what color should this explosion/fire be?" Using the Black Body material node, you can guarantee a realistic color to use on effects. To make the most of the Material Expression, it will help to have an understanding of Black-body radiation.
In this specific example, we use a Dynamic Parameter module within our Particle System. This allows us to create an arbitrarily named parameter that can then be used to drive some aspect of a Material. So we have a custom Dynamic Parameter named Temperature, whose value runs from 4500 to 0 throughout the lifespan of the particle.
When sent into the Material via a Dynamic Parameter Material Expression, it is used to drive the Temperature input of the BlackBody node. This means that the particle's color is going to reflect what a black body surface would look like as it dropped from 4500 Kelvin down to 0, or in more literal terms, from bright orange down to black.