If you do an image search for 'fire' you'll find some photos where it looks like the middle is red and the outsides yellow, some the other way around. Which you choose to paint is up to you. As a general rule the hotter the flame (usually the source) the brighter it is. As you'll see from the photos though the colours can all blend into one, so don't be too concerned about where your colours are going!
Start from white, and reapply it before bright colours if you have to (if painting yellow over red you'll get brighter colours if you put solid white down first). Tiny dots of any colour, including white, can be used to simulate embers, and you can finish with a little black at the edges and tips to show smoke, too.