Sorry to bang on about histograms (I'm not obessed with them, really!), but they have another major advantage that's worth mentioning: they are not affected by monitor calibration (or lack thereof).

Few people have calibrated their monitors accurately. Most who think they have probably haven't. So everbody looking at your four reference photos will be seeing them with a bias introduced by their monitor display and its properties.

A histogram is immune to those considerations - it reports only the data.

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