Fading can be done with several different methods, all with good outcomes.
You can use the White Wash method where you spread thinned white Artist Oil paint on the model, then remove most of it with a "Q" tip. Spray on an extremely thinned paint wash and build it up to a desired level or the Oil Dot fade where you apply several different colors of oil paint and then brush them altogether with a brush damp with mineral spirits.
I decided to try a version of the Oil Dot fade but with Pan Pastels.
I kept forgetting to get pics when I was working on the recent U33B and GP35 shells, but I did manage to get some shots on the last fuel tank I did.
I started out by painting the fuel tank with TCP Rock Island Maroon and then gave it a good coat of the Dullcote to give the surface tooth for the powders to grab on to.
In this case I'm using three colors of Pan Pastels: Yellow Ochre Tint for the fade, a rust colored Pan that is lighter than the Rock Island Maroon to keep the color close to the base color and finally black.
I start by dabbing my brush into the Yellow Ochre Tint and lightly padding it on the surface of the tank.
Next I do the same thing with the light rust color.
Then using a scrubbing motion, I work the two colors together on the tank.
Now the lighter colors tend to fade and disappear when you shoot Dullcote over them to seal it, so I apply the pans heavier than I need and start by misting multiple light coats of Dullcote. By doing this, the Dullcote is almost dry by the time it hits the surface and doesn't affect the Pans Pastels as bad as it would if you shot a full wet coat. If you watch closely you can see some the colors disappear, if this happens, simply re-apply another coat of the Pans as you originally did and then keep misting the Dullcote and let it dry between coats.
Eventually the Dullcote will build up enough to seal off the Pans, once this happens, allow the Dullcote to dry thoroughly and then comeback and shoot a full wet coat of Dullcote to fully seal things.
After that I hit the tank with black to give it a sooty or oily look and seal it with another coat of Dullcote. Eventually I applied some Tamiya Black Panel liner to give the tank an oily, greasy appearance.
This is how I did the fading on all of my recent U33B's and GP35's.