Well actually my whole second paragraph is about the purple :) The point of going from blue to white is to to visually convey that more pale colors mean 'closer

Well actually my whole second paragraph is about the purple :) The point of going from blue to white is to to visually convey that more pale colors mean 'closer to the baseline'. I.e., pale blue is 'slightly better than baseline', pale red 'slightly worse than baseline'. If you were to blend from blue to purple, that wouldn't work any more. But if you leave in the white in the middle, it looks as if there are 'no data' spots. Hence, use variation in intensities for non-baseline, different color for baseline. Cartography is not about conceptual consistency, it's about conveying information. I challenge you to make a map that conveys the message better than the choices made in the map in the OP (apart from subjective wah wah like 'blue is democrats' as was laughably brought up elsewhere in this thread).