Flames can be anything or color(s).
They are only limited to your imagination.
There are some rules to follow when laying flames down. To me you can go overboard such as having the flames all the way down both sides of the car. But this is just my $0.02 worth.
I've been collecting images of Classic cars for a CD I'm putting together and some of the images I've got are incredible paint jobs with flames on them.
One of my Classic car has flames on it that my buddy did when he owned the car. If I could change anything about the flames it would be they were buried under a clear coat. Why? Kids love to try to pick the edges of the flames with their finger nails. This means a lot of touch up for me.