Open up the material editor, change the lighting model to Unlit from the Details panel on the left. Then create a vector3(hold 3 and click on the empty canvas. When you double click on the node it will open the color palette) and set your color with it. Plug it to emissive and that should give you the basic idea of how it’ll work.
You can check the documentation to get further information and examples on materials(and everything else);
https://docs.unrealengine/latest/INT/Engine/Rendering/Materials/index.html