I could not find BrushConverter class in the latest Universal Windows App namespaces. Here is simple example to convert Hex string to Color class.

ColorUtil.cs

//using Windows.UI
class ColorUtil {
        // ColorUtil.FromHex("#FFAABBCC")
        public static Color FromHex(string hexStr) {
            hexStr = hexStr.Replace("#", "");
            return Color.FromArgb(
                (byte)Convert.ToInt32(hexStr.Substring(0, 2), 16),
                (byte)Convert.ToInt32(hexStr.Substring(2, 2), 16),
                (byte)Convert.ToInt32(hexStr.Substring(4, 2), 16),
                (byte)Convert.ToInt32(hexStr.Substring(6, 2), 16));
        }
    }