Discoloration

Does anyone have any tips for discoloration? It looks like this on both of my knees. Some days it’s darker than this. It’s not dirt. I lotion. It still looks like this. Any products or tips you recommend?

Edit: it showed up in the last 6-12 months. I don’t have any skin conditions or discoloration anywhere else. Could this still be a skin condition? What is it called?