The color of anodised niobium rings is on a layer of oxidation which can be scratched, a bit like a liver of sulphur patina. Sandpaper will rub the color away. Plier marks will remove the color. A soft cotton polishing cloth won't rub away the color. I do tumble niobium rings, mainly because the niobium rings can get a bur where they have been cut into jump rings. I have read that the color can fade with tumbling, I have never had rings fade and actually I think after tumbling the colors can be a little more intense because of the polishing. I generally tumble them only for about 30 minutes with stainless steel shot and a burnishing compound. For any rings that you are unsure about tumbling, just make a short chain and try it before you test it on something you have invested time in.
The ring on the left has had some of the color removed by fine sandpaper. The ring on the right has a silver mark where the pliers marked the ring.