Cold if baby will take it. Its easiest
When I started the transition I started with warm milk in a bottle so it was as similar to formula as it could be, then transitioned to cold milk in a bottle, now she drinks it cold from a straw cup. Only took maybe 2 weeks or less
We've been giving him room temp since he started on formula. He actually won't even drink his formula warm. I hope someone can give you some tips ❤️