Lighting and shading
We need to introduce a light source into the scene and provide a shader that will use it. For this, the cube needs additional data, defining normal vectors and colors at each vertex.
Vertex colors aren't always required for shading, but in our case, the gradient is very subtle, and the different color faces will help you distinguish the edges of the cube. We will also be doing shading calculations in the vertex shader, which is a faster way to do it (there are fewer vertices than raster pixels), but works less well for smooth objects, such as spheres. To do vertex lighting, you need vertex colors in the pipeline, so it also makes sense to do something with those colors. In this case, we choose a different color per face ...