If there are no -1/-1 counters on it when the triggered ability resolves, the ability does nothing. There is no penalty for not being able to remove a counter.
This creature enters with four -1/-1 counters on it.
: This creature gains shroud until end of turn. (It can't be the target of spells or abilities.)
Whenever you cast a green spell, remove a -1/-1 counter from this creature.
Whenever you cast a blue spell, remove a -1/-1 counter from this creature.
If you cast a spell that's both of the listed colors, both abilities will trigger. You'll remove a total of two -1/-1 counters from the Hatchling.
