Can you refill or add cold water to an electric urn on Yom Tov if the urn was plugged in before Yom Tov?

Yes, but the on/off/heat level switch should be covered. Normally this is not a problem as there is no need to adjust if the water is hot, but if you put in cold water there is a chance one would fiddle. This is all regarding Yom Tov, on Shabbat no water can be added at all. What if there is an electric light?