you can actually wash in what ever temperature water you want.
You just make sure your final rinse before you get out of your shower, or your final rinse washing is with cold water.
Cold water does indeed seal the cuticle, locking in the good things you've moisturized your hair with, Cold water gives more bounce to your curls, if you're a curly head, and cold water gives more shine to your hair, if you're straight haired.
Oh yea. washing your hair in hot or warm water is good, just because the hot water opens up the cuticle first, to let the good stuff in, if you're conditioning your hair with good stuff, and it also makes it easier to wash the dirt and build up out.
Just make sure the final rinse is in cool or cold.