A SHADY BUSINESS
As an absolute minimum, I employ three shades of weathering paints. Many are branded with useful names, such as Frame Dirt or Weathered Black, while others can be chosen purely by sight. Basically, we need an earthy brown, rusty red and various dark greys, along with a matt black. Each of these can be inter-mixed to fine-tune the shade to suit your own tastes and the demands of the model.
Railmatch enamels have long been a favourite of mine and my usual palette consists of 402 Frame Dirt, 403 Roof Dirt, 412 Weathered Black, 405 Dark Rust and 419 Matt Black. With these five colours, all manner of locomotive and rolling stock projects can be tackled with surprising variety of tones achievable by inter-mixing.
It can be convenient to thin a small number of weathering shades before work commences, allowing you to switch between the tones without interruption. Thin to the usual consistency, using the technique explained in MR260.