Why does my water in my hydrogen fuel cell turn to a weird color?

When I turn on my hydrogen fuel cell or HHO fuel cell and it runs for a little bit, the water starts to turn like yellowish brownish. And it starts to form bubbles on the top of the water that turn really dark brown or black. What kind of water do I need to use and if I need to mix something in with the water, what is it?
Thanks Dave I didn't want to mention that i was using tap water because I wanted to see if someone would state that could be the problem, but when i use distilled water do I need to mix anything in with it. Or can I for better results?