Think of your light source as a bull’s-eye or target with your most brilliant color being the light itself. As the light travels from the source it will get darker. So white to yellow to red then fade to black. The intensity and size of the light source greatly affect the size of the bull’s-eye.
The bottom of the robe should be very, very bright. Like the fire itself, as parts of the model get further away go to orange then red, then purple. The areas in shadows should be black -or whatever mood color you are using, such as dark purple, green, red or whatever. This area is very dark, you could lightly highlight this area in order to give the model components definition. Don\'t paint the rest of the model as being lit from overhead!
Please refer to the dwarf cannon model earlier in this article, which is done really well by the way. Notice how everything not lit directly by the light source is very dark, this is because of the harsh lighting of the muzzle blast. Your scene is very similar, one intense light source. The back of the cannon is like your model on the pyre, there should probably be very little of the model being hit by light. Mr. Shawn\'s reference to imagining a paint bomb going off is a good analogy to figuring out where the light is going to hit.
One thing that you can do is take a similarly shaped model hold it upside down and shine a flashlight from above. You should be able to see where the light is hitting the model and use this as a guide. You might have to fudge some additional lighted areas on the model if there are too few in real life. This might help the model read better in this case.
Let me know if you need more illustrations to help clarify what I\'m talking about. I tend to ramble on a lot.
Good luck!