Noxious Hatchling comes into play with four -1/-1 counters on it.

Wither (This deals damage to creatures in the form of -1/-1 counters.)

Whenever you play a black spell, remove a -1/-1 counter from Noxious Hatchling.

Whenever you play a green spell, remove a -1/-1 counter from Noxious Hatchling.