Chalks work very well when learning to weather, as you can built them up gradually and if you don't like what you see, you can wash them off and start over. Once you like what you've done you can set them with dull coat or similar product. They do lighten up when over sprayed to set them. Many of the pros use a combination of chalks and paints.
I agree with Hot Water, have a picture (color preferred) handy of a particular car you like to guide you.
