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Hybrid consideration

Interesting. Does give pure white red-looting, on a green trigger. Which is a stretch. But probably not a disasterous one.

Green cares about big creatures or creatures in general; caring about small creatures is not a specifically green trigger condition as much as a white trigger condition.

Compare e. g. Bygone Bishop.

I meant more "Triggers on having creatures".

It doesn't "trigger on having creatures" it "triggers on having a specific sort of creatures".

If the specific criteria was "Zombie creature" it would be a black trigger (in general, depending on the environment also blue etc.), if it was "creature with flying" it would be blue (in general, environment-dependent etc.), but if its "creature with mana value 2 or less" the specifics push it towards white, not green.

But even ignoring all of that, "triggering on having creatures" is not so much a green thing that it should be considered a green trigger on a white card - specifically since white is the creature color caring about "many" rather than "large", so it is more inherently white to trigger on creatures entering the battlefield since it will happen more reliably often for white.

Hence Soul Warden. Hence white has (varying depending on the actual variant(s) of the wording you search for) 30%-50% more cards that trigger on any creature entering the battlefield than green.