The answer to 'why is my shut-off valve rusty water' is that a rusty shut-off valve is often caused by mineral buildup or corrosion within the valve. Over time, the metal components can deteriorate, allowing rust and discolored water to come through. This is a common issue with older valves or those in homes with hard water.
Some common symptoms include difficulty turning the valve, leaks around the valve, and discolored or smelly water coming from the valve. To fix this, you may need to replace the entire shut-off valve, which is a job best left to a professional plumber. They can properly shut off the water, remove the old valve, and install a new one to restore proper water flow and prevent further issues.