Creatures are inconsistent on when their power or toughness is colored green

It seems like green coloration should be indicating "this has been raised above the base power/toughness".

But it's used very inconsistently, even among similar creatures.

Brushfire Elemental always colors both power and toughness green when landfall triggers.
Akoum Hellhound colors only toughness green.
Canopy Baloth colors neither green.