Material vs Decal in Live Rendering (CCS-OC)

Hello all designers. What is the right approach with work in "emission picture", like Traffic light. Ofcourse I don't want modelling physical lamp, only picture.
In Decal we are able to define emission texture but I don't see any sets in Definition. In consequence I can't change value of this emission.
In Material I have all stuff to definie emission of picture/texutre.