The original ghost flames, from my understanding, were done in murano pearl, also marketed as "Glint Pearl". I have one job I've done with them on a '40 pickup. The problem was that they disappeared if the light angle was not right. On a curved body, like a '40 Ford, that means you only see pieces of flame.

Today, like most painters, I use a shade of paint similar to the base. At least then you can see the whole design.