I am painting my Chaos with an orange theme. Granted I am using the old colors (Solar Macharius Orange)
Don't know the conversion to the new colors but this is what I use.
Base coat with Calthan Brown
Layer with Solar Macharius Orange
Wash with Ogryn Flesh
Layer highlight with 3:1 ratio of Skull White:Solar Macharius Orange
If I was painting the Pyre however, I would use a yellow base. Something that covers well, like a base paint or if you can get your hands on an old foundation yellow.