Compared to the white sphere the internals of the ray tracer are a good deal more sophisticated by here:
There is still no surface physics other than a very simple hard coded shading model — the colour on the spheres all comes from the colours of the lights.
This extra complexity, both in terms of the models and the capabilities mean that this render is about 25 times slower than the last. See the code at Scenes/early/five-spheres-coloured-lights.cpp.