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I don't understand one thing: why don't creatures of the same faction have consistent coloring for their upgrades and alt-upgrades? Tier 1 upgrades have a blue tint, but already in tier 2, the blue tint is only for the alt-upgrade. There was more order in this regard in Heroes of Might and Magic V: when you look at the Sylvan faction, it's immediately clear – the gold unit is the upgrade, and the blue one is the alt-upgrade.
Added 3 minutes later
Well, maybe it wasn't perfect in Heroes of Might and Magic V either – the upgrades for drakes and unicorns don't match the overall palette, but at least the units were initially designed without alt-upgrades.