Type of light return you see depends on the stone's proportions, faceting, and the type of light there is around you - in some types of lights like spotlights in restaurants and elevatosr all diamonds show coloured light return, in other types of lights like those diffuse office ceiling lights all diamonds will be white and you'll see facets changing from white to black back to white as you rock the stone, and in "mixed" types of lights that aren't either of these extremes different RBs can err toward more white or more colour depending on how they're proportioned. I've owned RBs nearing (but not yet at) both extremes and the difference in personalities is visible if you're looking for it - neither is objectively better than the other, just different. My personal preference is for more coloured light return, even though it's at the expense of more white light return
Lots of reasons why - it all kinda works together. When white light refracts into the stone it separates into its component wavelengths, and what happens from there depends on how the stone is proportioned and faceted - some things that can make a difference (there are books and hundred-page threads on here on this sort of stuff) : greater angle difference btwn pavilion mains (the arrowshafts) and shorter LGFS (the pairs of triangular facets between the arrowshafts) allows the separated wavelengths to reflect internally at more different angles (decreasing odds of recombining into white)... longer mains are shallower, closer to the angle of the mains, and they extend further from girdle to culet so there is more physical main/LGF facet meet area. High crowns mean larger crown facets that are angled further away from each other so exiting dispersions leave the stone at very different angles, increasing your eyes' odds of catching a single dispersion (and seeing a single colour). The fatter arrows (shorter LGF) are larger facets that can reflect large, slow, bold bolts of light, rather than skinnier arrows that yield a twinkly, "splintery" sort of light return..