I have read of these magic powders in aircraft, train and military modelling magazines, but never used them. When I have wanted to add mud and dust I have scraped off dust from artists' pastels and stuck it to vehicles with thinners, as below on a White Army Whippet:
But there are two problems with this method, firstly it is a fiddly nuisance, and, secondly, most of the dust still ends up on one's hands. So, for wargames models I have tended just not to have bothered, as here:
Three Frontline T34s. But, even given they are for games, they still look a bit bare - I certainly don't like the tracks in this view! So, the task is (as 'Mike the Knight' says), build:
and use the Mig pigments.