Nevermind. I finally remember where I saw it. Here it is for anyone that wants to know.
"The burn effect on the flaps. Okay.
You have to paint backwards. Outside first to the middle. I used Boyd's candyapple colors. They are trasparent when applied thin in an airbrush.
Brass/yellow stripes first on the outside like a "V" Then hot pink further towards the middle, then teal, blue, then purple and in the center matt black.
I have a screen used Rocketpack and that is how it looks...it took a bit to figure it out! Each flap of my pack is different. All the Disney pack parts got jumbled and swapped around at some point
Same treatment on the engines...but you paint the engines all dark metal color and then paint a layer of silver everywhere you want color. Then paint over the silver with your color. Since the color goes over the silver and the black you get that "I got really hot" look."
Hope Rocketbobs doesn't mind I posted this.