There is another, less time-consuming way to change the look of your fireplace mantel though. You can paint it using a heat-resistant enamel paint. A black paint will give you a look that is close to the original slate, but there are many colour options. Likely, you won't choose purple!
First, lightly sand the fireplace mantel. Wear a dust protection mask while sanding. This will help the new paint to grip. After brushing away all the dust, carefully paint over the purple using a heat-resistant enamel paint. (It should say on the tin that it is formulated for fireplaces.) Applying this type of paint takes skill because it dries quickly and runs can be quite noticeable. Try to make sure that you smooth out any runs with the paintbrush. Also, wear protective gloves and ensure that you have good ventilation. Follow all safety instructions on the tin of paint. When the paint is dry, you will have a smooth and shiny surface and best of all, no more purple!