Solution Finish works great. They have both a Black and Gray. You might also want to try CarPro Dlux Coating. It darkens up plastics and locks in the seal as it's a penetrating coating on plastics. I usually try Dlux first as it's accomplishes both a restoration and coating in one. A few drops goes a long way. If it's too far gone or if the customer isn't paying for a coating, I use Solution Finish.