Whenever you cast an instant or sorcery spell, Spellrune Painter gets +1/+1 until end of turn.

Daybound (If a player casts no spells during their own turn, it becomes night next turn.)

Whenever you cast an instant or sorcery spell, Spellrune Howler gets +2/+2 until end of turn.

Nightbound (If a player casts at least two spells during their own turn, it becomes day next turn.)