Good tip for weathering: get a set of watercolour pencils (i.e. colouring pencils that are water soluble, find them in artist's material shops)
You can use them in a number of ways; First, you can lightly draw with them on your pristine paint for a rough effect, second you can do the first one and them adjust the effect by using a sable brush dipped in water, third, you can scrape small chips of the coloured pencil onto a saucer with a scalpel and mix it with water to get some watercolour paint, and it is a very flexible way to mix the right colour. The advantage of all this, is that if you don't like what you've done, it will wash off. Fix it by using spray fixer, or if you are cheap, hairspray will work too (use it lightly or you'll get a satin sheen, which you may not want). Try it on something scrap first to check the technique out, but I think you'll like it, much easier than pastels, and easier to correct too.
Chock