dirt on rally car

Well, it depends on which way you want to go. Which rally you want to replicate and if dust or mud is applicable for the rally.

Above is great for dust, the Tamiya weathering set will do great. For mud, if you don't want to use a/b you can use the weathering set but also create "droplets" with an old toothbrush and paint - simply flicking it with your thumb. (Pull back with your thumb over the brissles and let it flick forward naturally...) Experiment a bit till you get the pattern and it should not be an issue. 

Word of warning - I often use set D for other weathering, but assume the others are the same - they tend to disappear when you touch it. So do the droplets first, let it dry and do the smuding/smooth mud later.

Look into some weathering pigments, too. Tamiya has some, they come in 3 colors per set and there are several different sets. I've never used them, but I've heard good things about them. Mig pigments are supposed to be really good, too. The Doog did a tutorial on here in the armor forums about Mig products where he weathered nearly an entire vehicle with them. There are also some other brands of pigments, but those are the 2 most well known
I'm pretty sure that all of the pigments can be applied the same way, with a dry paintbrush. They can also be mixed with water or alcohol to make a thicker mud-like goo.

Also, there was a Scale Auto article a while back that had a nicely weathered rally car in it, I'll have to search through the collection to find it though. It had some good tips, you may want to email Scale Auto to see if they can find it and if you could purchase it.

Hi there you can use tamiya "buff" not to sure on the colour code but spray it on with very fine mist coats and stop when you think it looks right.

Mike