Add a variable to the blueprint you want to control the Niagara System variable (in this case, Linear Color). Make sure to give the variable a default value and make it Instance Editable so that it can be manipulated in the blueprint actor properties screen in the level editor.

Create a new User Parameter for the Niagara System in the Niagara System control node.

Bind the new User Parameter to the module of the emitter you want it to be on by going to Link Inputs > User > NameOfParameter. In this case, we will replace the Color of the Initialize Particle module with the system parameter "MateriaColor"

Setup the variable to be set in the Construction Script (or Event Graph) of the blueprint. Do this by getting the Linear Color value and the Niagara component (system). Drag a "Set Niagara Variable" using Linear Color. The InVariableName input of the Set Niagara Variable must match spelling and is case sensitive to that used in the Niagara System User Parameter created and set in #2 above.