In the "old days" they'd take a big pickle jar (one gallon) and fill it with scraps of yellow, orange and red gels. Maybe even throw a small piece of blue. This would be rotated in front of a light. Gels, in case you're not theater oriented, are sheets of colored cellophane in different colors to color or"gel" theatrical lights. Buy them from any theatrical supply or Roscoe.
These are crumpled slightly and added to a jar til it's about half full. You could attach the lid to a motor, with a light source behind it. Project it up into the logs or onto a wall.
We built a 3 ft. high pile of logs for the witch burning with a large industrial fan underneath, on LOW. Small strips of white silk or similar material, were attached to the logs and blown by the fan, illuminated by yellow lights. A fog machine finished the effect off.
Feeding the fog near the fan blew the fog up and around the witch. All of these effects combined looked really good. The jar of colored gels was projected behind the pyre.
Have an actor in burnt make-up or good mask come out of the burning pile of logs toward the audience if possible. Good scare while they are distracted looking at the fire effects.
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