A little vague but do you mean right clicking on a node in the material editor and selecting convert to parameter then selecting it, details panel and settings its name to text like mainColor and then in your BP you create a dynamic material instance of it and then setScalerParmaeterValue or SetVectorParameter value or whatever type of parameter it is (it will tell you when you select the node)
This should get you where your going.
Hope this helps
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