Possible explanation for Egyptian pyramid art work without LIGHT

Just something I noticed on the Ancient alien tv series regarding the Egyptian pyramid art. It was proposed that a form of lighting would need to be used to work on the walls within the pyramid. Why? It is possible that the art was all done on the walls of the rocks and then put into the place. This would mean all aspects of the pyramid building could be planned in advance and merely placing the blocks into place like lego. Just as another theory.