You don't need to own the wash, I used thinned paints for all my washes and they work beautifully. Just mix the colour you need from what you have.
One thing you might want to consider though is that fire is its own light source, so highlighting it in a conventional way (washing/drybrushing) doesn't make a great deal of sense. It might make more sense to paint it light first and then dry-brush it with darker tones to show the heat coming from within. Or conversely run a lighter colour into the cracks with some water.
EDIT:
I found this image of mario throwing a fireball which I think illustrates what I'm talking about quite well. As you can see the ball in the middle is hot and white, but the outside is cooler, going from yellow to a much darker red at the very outside edges. This is the opposite of how a solid object being lit externally might look.