There's not much to it: You create a material named "Emissive" (exactly) and assign it to the faces that should change color. Then you UV-map those faces to a completely white part of a texture. I normally use the upper right corner of Keen's Textures\Models\Cubes\ConveyorTexture_de.dds for that.

Everything else is handled by the game logic Keen implemented for status-changing blocks.