Error System.Windows.Media.Color.A.get

我有一个项目，这是一个图像编辑器和我错误地参考市场的System.Windows.Media.Color.A。get'，而不是因为我犯了这个错误，这是品牌我在Visual Studio 2012中的错误代码是为windows phone 8

非常感谢您的帮助

        public WriteableBitmap Process(WriteableBitmap input)
    {
        var result = new WriteableBitmap(input.PixelWidth, input.PixelHeight);
        var resultPixels = result.Pixels;
        var inputPixels = input.Pixels;
        var ta = Color.A;
        var tr = Color.R;
        var tg = Color.G;
        var tb = Color.B;
        for (int i = 0; i < inputPixels.Length; i++)
        {
            var c = inputPixels[i];
            var a = (byte)(c >> 24);
            var r = (byte)(c >> 16);
            var g = (byte)(c >> 8);
            var b = (byte)(c);
            int gray = (r * 6966 + g * 23436 + b * 2366) >> 15;
            a = (byte)((a    * ta) >> 8);
            r = (byte)((gray * ta) >> 8);
            g = (byte)((gray * ta) >> 8);
            b = (byte)((gray * ta) >> 8);
            resultPixels[i] = (a << 24) |
                (r << 16) |
                (g << 8) |
                b;
        }
        return result;
    }

var ta = Color.A;
var tr = Color.R;
var tg = Color.G;
var tb = Color.B;