"open" doesn't sound right. If the cold water was already running I would say "turn up". If it was not already running I would say "turn on".
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shininggirlAnd so to close it you say "turn down". Right?"turn down" means lessen the flow; "turn off" means stop it completely. "shut off" can also be used.
shininggirl In a bathroom can I say "open the cold water"? or should I say 'turn up" the cold water?turn [on / off] [the water / the light( s) / the heat / the oven / the TV / the computer]