Yes, the potion_overlay.png is the filler color for potions, and changes normal, splash, and lingering potions.
If you want to use a different overlay texture for splash potions, change it within "/assets/minecraft/models/item/splash_potion.json", and for lingering potions, go to "/assets/minecraft/models/item/lingering_potion.json". You should be able to figure it out form there