If you turn off the valve, no water should be coming out (when it's on the side which you turned off), FWIW.
Also, regarding whether there is a mixture of hot and cold water, it depends on the specific sink.
If you can move the handle to hot or cold, before opening the sink, then whatever you set it to, that's what will be, it won't mix.
However, if it's a sink where you can only move the handle to hot or cold, after you open it, then, as soon as you open it, it will be mixing the hot and cold... Even if you will change it right away.