If the humidity is good, you don't have to water it. If it's low, you can add water, just don't go overboard. You never want it to be damp (I would consider damp like if you wet a paper towel and wring it out), but somewhere between that and bone dry. You will never have the exact same humidity across the whole enclosure and that's fine. Color of the substrate is something after some practice you will be able to tell when it needs more. The hot spot will generally be a lighter shade (less damp) because more has evaporated on that side.
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