Right here's a quick walkthrough of how i do rust
Heres the main colours that i use, Raw umber (very dark brown), Black & three rust shades as well as various shades inks & washes in brown, red, orange & green.
I start by mixing black & Raw Umber & paint an area that i want rusty, then highlight with Pure raw umber. I will then use a sponge & lightly sponge some extra patches of rust onto the model, these i may add another highlight to with a brush, but i tend to leave them..
Then carry on highlighting mixing the other colours in
Till the desired effect is reached
Then add various washes & glazes to blend in the rust.
The next stage will be the weathering powders.
Hope this helps so far.