I know what you mean mate. I used the rust wash from Humbrol on my sailing trawler, it looked too brown whereas real rust has varying shades but the overall colour is more orangey than it is brown. Having said that the wide range of washes they have now can be mixed to get the shades you want and flow really easily when applied with a brush, its almost like applying a coat of water!
But check out acrylics (paints and washes) from MiG and Vallejo ( I'm slowly moving to vallejo) They can be applied over other paint substrates like Humbrol or car paint, even better once dry they can be overpainted by other mediums like Humbrol and other modelling paints, as well as artists oil colours.