A quick update.
The models I'm working are Game Workshop gaming models that I paint to a high standard (high standard for me anyway!), and occasionally use for playing Warhammer 40,000.
The best yellow I've found is actually a Krylon Fusion spray paint called Sunbeam Yellow. The only issue is that it's gloss finish. Once I get a good base coat of yellow on the model I need to seal it with Testors Dullcote to get a good matt finish I can weather from there.
I'm about to start on the vehicles, but here is an example of the yellow with a Raw Sepia + Black Wash applied. I'm very happy with the deep yellow so far. I am using weathing powders and pastels to dirty down the yellow and am also carefully painting on chipped areas.
When help up to an actual worn and rusted back hoe blade, (i.e not a model a real back hoe) the yellows are the same.
I'm looking forward to working on the vehicles that I will start work on in with in the next couple of weeks.
Note that these are work in progress pictures and don't have any weathering powder at this stage.
http://www.flickr.com/photos/80523470@N00/4832184795/
http://www.flickr.com/photos/80523470@N00/4832184887/