That's quite an interesting graphic. It could be something to do with my monitor/setup, but some of those colours are near-indistinguishable, while others are totally unique and isolated. RAL 6019, slightly south-west of the middle, is an example of the latter: quite unique in this chart, no other colour being remotely close; whereas on my monitor at least, there's near enough no difference between, say, 6010 and 6025, or 8012 and 8015.