There are some things you should take into account:
Before applying the wash, make sure you have a gloss surface, there are many ways to obtain this: gloss coat, future, clear top coat gloss... Never wash on a flat surface! (here I dissagree with the post above) the dull coat finish will be instantly stained by the wash and you won´t be able to clean it.
Use different paints for top coat and wash. This is to avoid reaction (dilution). If you used light desert tan enamel you should use oil paints or acrylics for the wash. Use of course the appropriate thinner to remove the wash excess.
If for any reason you want to use both top coat and wash color enamel or acrylics then seal the top coat with future (maybe two hands). If you're using both acrylics don't clean the wash excess with acrylics thinner (it will dissolve the future and ruin your base color too), use water instead.
I use both enamels and acrylics for top coat and only acrylcs for washing using the sludge wash technique.
Good Luck