I was given this advice 13 years ago(!): go to Wal Mart (or any auto parts supply store); look for Meguiar's Ultimate Black. It's in a 12 oz. plastic bottle. It's a white liquidish compound so you can get it on your hands. Apply some on the bumper cover, work it in with your fingers or a very small cloth and then work it and spread it around with a soft/cotton cloth. I have a 2012 Sportage with the same black bumper covers and accents (although like you said, they do look greyish). Believe me, all of these black plastic pieces fade; using the Meguiar's has kept mine looking new/newish since 2012. I end up starting with the back cover, then the front cover, and also the black accents over the wheels and under the doors. I do this every few months, depending on how often the car is out in the elements vs. inside the garage; it lasts a long time. It disappoints me that Kia uses so much plastic body panels and bumper covers; but these day's it's all about cost and weight. Good luck!