"The milk is lukewarm."
Translation:O leite está morno.
É is usually used to describe things that have a permanent status or unchanging state, while está is for things that have a temporary status or could change at any time.
For example, you say "Sou alto" (I am tall) and not "Estou alto" because your height can't change from tall to short.
Warm milk can and will eventually change as it's temperature drops and cools. So, está is more appropriate.