Simple guide on how to change a particle systems color
First go through all the emitters turning them on & off 1 by 1 (see image below) to see which ones you need/want to change.
Turn off all except the one you want to change and select the Color Over Life module on the active emitter. (Some particles may have initial color instead of color over life. Both edit the same)
You can click the "S" by the checkbox to turn off all other emitters and "Solo" the selected.
Expand the "Distribution" elements in the "Details" tab.
Change the values in the "Points" to adjust color. Some particles may have more or less "Points" & different parameter types. They all edit the same way for the most part.
XYZ = RGB
X & Y where changed on all 3 "Out Tangent" points to make the blue color.
Rinse & repeat above steps for each emitter you wish to change.
Different values in different tangent points = different effects & colors. Play around with the numbers & tangents to achieve your desired colors & effects
Remember to re-enable the other emitters you want active when you're finished editing..
For more detailed information on particle system emitters and modules, please visit the UE4 Particle System User Guide