When using pastels experimentation is the key.
For exhaust staining I use a combination of black and white (I've never seen a grey pastel) and apply them alternately to get a better 'contrast'.
For a dusty effect mixing white and various earth tones do the trick. To simulate mud I mix pastels with gloss clear, then blend it all together with dry pastels for a more realsitic effect.
I have never taken the base colour into consideration when using pastels as I use them for weathering only.
If you wanted to simulate sun bleched paint on an a/c or armour model, it would be best to lighten the colours you spray on the model gradually to reach the effect you want.