A couple clarifications (because I simply winged it in one take and without thinking it through):
1. On my light grey UGG slippers, I first did 3 rounds of cleaning the entire surface using the UGG Cleanser--first with a wash cloth, and then twice more with a sponge (with letting them fully dry in between), but they still looked terrible. [A few of the stains were from splattered grease while I was cooking.]
2. On the eggplant UGG boots, I did 2 rounds of distilled white vinegar on the entire area followed by drying fully, which only seemed to help a little.
3. I saw an article somewhere online about using chalk or baking soda on a FRESH water mark to basically dry it out to prevent a dark water stain. Of all the videos I watched and articles I read, no one had tried chalk on a DRY water or other type of stain.
To be clear: this doesn't REMOVE stains, but hides them, which is good enough for me! If I had thought to try this first, it would have saved me TONS of time. If it doesn't work for your UGG footwear for whatever reason, please don't be a hater--I'm just trying to help!!!