when painting desert camo, do you applied wash?

yes, for mordern sand coloured vehicles a yellowish brown wash works pretty well. I use several oils washes in the brown to yellow spectrum to add layers of dirt and depth. There is plenty of dust and dirt in the desert but it rains pretty hard too so any vehicles that have encountered winter/spring rains will show streaking and such. Cheers.

what's the ratio?to much brown,and desert camo look brownish.

very little paint to a lot of thinner, like 10% paint to 90% thinner. I use artist oils so a combination of umbers, sienna and ocher usually work, but again keep it thin and wipe off excess quickly.

what color you use for dry brush?I experiment with white and flat yellow,But I am not satisfied with the result.