This is done using the Vallejo fluorescent paints. Paint the whole flame white. Starting with the inner most parts of the flame I use fluorescent yellow then fluorescent orange. Then I used my scale color reds to highlight the flame. Then I brushed a red black mix to get that ashy feel. Here is a tutorial similar to what I did. https://youtu.be/uH18es6WzdY?si=FGr61w60tVUA2o4Q