It looks like the normal font render engine does not support this. You could use Pango for this:
It seems the fill color is ignored and you will have to use the 'span trick'. I will try to figure out why the font color is not working when I have some spare time.
MagickImage label = new MagickImage(); label.BackgroundColor = System.Drawing.Color.Black; label.FontPointsize = 50; label.Font = "Arial"; label.Read("pango:<span foreground=\"white\">" + "مرحبا بكم" + "</span>");