Diffuse color boost is on the material calculation, not the light. The rays are just bouncing around more accurately now and distributing their energy around the scene better. The equation for color boost is simple: Color ^ (1/Boost). If you go too high in boost, like using four for instance, you’re going to blow things out extremely badly by raising something to the power of 0.25(fourth root). A value of 0.5 becomes 0.84. Even the documentation recommends avoiding going above 2 because “It works best to keep the value below 2 as it also causes reflections to be brighter than the scene.”
Other than the exaggerated second image with color boost 4, it all of the images in your two posts look far more accurate in the 5.6 versions, in terms of the light energy levels being bounced around, but again, you’re driving the boost too high and causing some ugly discolorations like in the narrow corridor shots in the second post… In instances like these, you need to lean more into exposure tweaking, rather than trying to artificially color boost things.