When the Background / Foreground property of the RadBarcodeQR control is set, at runtime time that property is ignored.

Workaround: 
On loaded, change the pixel value  of the ImageSource of the code in the following manner
(in case the Foreground is set to a valid SolidColorBrush value):

public void RadBarcodeQR_Loaded(object sender, RoutedEventArgs e)
{
	Image image = this.ChildrenOfType<Image>().First();
	ImageSource src = image.Source;
	Color c = ((sender as RadBarcodeQR).Foreground as SolidColorBrush).Color;
	image.Source = ChangePixelValue(src, c);
}

public static WriteableBitmap ChangePixelValue(ImageSource src, Color c)
{
	BitmapSource originalSource = src as BitmapSource;
	WriteableBitmap modifiedSource = new WriteableBitmap(originalSource);

	int h = modifiedSource.PixelHeight;
	int w = modifiedSource.PixelWidth;
	int[] pixelData = new int[w * h];
	int widthInBytes = 4 * w;

	modifiedSource.CopyPixels(pixelData, widthInBytes, 0);

	int colorInt = BitConverter.ToInt32(new byte[] { c.B, c.G, c.R, 0x00 }, 0);
	for (int i = 0; i < pixelData.Length; i++)
	{
		pixelData[i] ^= colorInt;
	}

	modifiedSource.WritePixels(new Int32Rect(0, 0, w, h), pixelData, widthInBytes, 0);
	return modifiedSource;
}