raw sienna could work, but as mentioned, I just stick burnt umberburnt sienna
for example the rust here on the hull was done just by placing a little bit and then dragging it downward with the brush and using some thinner where it got too dark
I use real rust, too.
I use steel wool scouring pads in the kitchen. When the soap is gone, and they start to rust, I put them in a small plastic cup and pour water over them. Through a couple of cycles of evaporation, the steel wool rusts and leaves a powder. I grind this with a glass stirring rod to get it as fine as I can. Then it can be applied with a brush, like powdered pastel chalks.