I use warm water to wash and initially rinse, then finish off with a cool rinse.  The cool water will close the scalp and help prevent oil production.

When I'm taking a shower for my all over  body I use hot water but for my hair only I use luke-warm for washing, and the coolest that I can stand.

No, using the coldest water is wayyyyyyyy ok!  I'm not sure if you've heard this saying from a famous female actress.  She is the most beautiful actresses in the world!!  Her name is Jane Seymour, and her hair beauty secret to healthy shiny hair is that she washes her hair in cold water.  No warm or hot water at all.  She washes it in the coldest water that she can stand.  She says that the cold water closes up the hair follicles and hair shaft, and it calms down the  frizzies, and gives your hair a  beautfil healthy shine.