// Instantiate the Presentation class that represents the presentation file
using (Presentation pres = new Presentation("presentation.pptx"))
{
    // Set the background color of the Master ISlide to Forest Green
    pres.Masters[0].Background.Type = BackgroundType.OwnBackground;
    pres.Masters[0].Background.FillFormat.FillType = FillType.Solid;
    pres.Masters[0].Background.FillFormat.SolidFillColor.Color = Color.ForestGreen;

    // Save presentation
    pres.Save("SetSlideBackgroundMaster_out.pptx", SaveFormat.Pptx);
}