I hope this helps!
-I start by using a chocolate brown exterior spay paint in the rattle can meant for painting over rust and paint the wheels faces and the truck side frames. I give it 24 hrs to dry and airbrush with a 50/50 mix of Dullcote and lacquer thinner.
-The next step is to apply with a makeup sponge, Winsor Newton Artist Gouaches, raw sienna, burnt sienna, raw umber, burnt umber and white. Start with the lightest colour first working to the darkest with a coat of Dullcote between colours. Gouaches are opaque water colours in a tube.
The trick is to get the sponge to the right moisture content using window wash fluid from your car to moisten the sponge. When you think that its just right wet it some more and then blot the sponge between two pieces of terry cloth towel or old sweatshirt material.
-Next is to load the sponge, I use a piece of ceramic tile and squeeze out about 1/8th inch of Gouache on the tile and dab the sponge into the colour. Then keep dabbing the sponge on a clean spot two to four time to even out the colour and then dab it on your wheels. It's almost like dry brushing but with a sponge. Because the gouaches are water based, if you don't like what you done with that colour and you haven't fixed it with dullcote, you can wash it off with a damp sponge and start again.
-Once you've got the combination of colours in place and you are happy with the results, dullcote it and give it a very light touch with your sponge loaded with white a just barely catch the edges for high lights.
I've attached these photos to give you an idea of what it looks like finished in HO scale.