My research on the color of kaftan's of the Polish Hussars was that there was no set kaftan color and they were found in a wide variety of colors. The Knotel print shows a Hussar with a blue kaftan while an officer in the background is in a red kaftan. Other colors I've read of were blue, green, brown, yellow, buff and of course red. It is possible that wealthy commanders may have dressed their troops in a single color, but having a variety of colors does not seem to be incorrect.
Terry