MrDWhitey beat me to posting the link. The description there is pretty good. And it is a common mistake, not just for first time fire painters either. If you dig around the GW site, some of the 'eavy metal team get it wrong on occasion.
If you want to look at some pictures of fire to see how it actually looks, it's going to be much easier to see the colour pattern in a bonfire as compared to something small like a candle.
What you can see in a candle that isn't addressed in the fire theory link is that the very bright base of a fire doesn't always start at the very base of the fire. It depends on the fire size and source, but often the area of the fire source it is darker. You can see the dark combustion material (wood for a bonfire, wick of candle, rag of torch, etc...) through the flames. You can see this very clearly with a candle flame, and it may be part of the reason flames are painted backwards.
The light yellow/white area of the flame sometimes starts partway up on the combustible source or even a bit above the source. Again this depends on the size/type of the fire. In your case, i wouldn't expect the lower windows in the sconce (is that the right word) to be bright yellow/white even though they are at the base of the fire. They would be a darker colour, possibly highlighted to a yellow/orange, then 'highlighted' to an orange red.
Also, it's not really emphasized in the article, but if you look at the pictures of the fire, it's not just a gradient of white at the bottom progressing to red at the top. It's also white in the centre of the flame progressing to red on the sides and the top. The sword in Marcus Leitdorf's post shows this pretty well.
Edit: yeah, it's a long post - i was bored. Hope it's helpful to someone
