Applying custom materials to 3D Tiles isn’t currently supported. What you can do is use the 3D Tiles styling language to change the color and opacity: https://sandcastle.cesium.com/index.html?src=3D%20Tiles%20Feature%20Styling.html. In the case of point clouds, that does make it possible to color by elevation: Precision of POSITION_ABSOLUTE point attribute for 3D Tiles styles for point clouds but that won’t help in the case of photogrammetry.