Basically we use my old junky chalk and either acrylic or tempera paint. The students then dip the pieces of chalk into the paint and apply to the paper. Since you can't put a lot chalk on paint it causes short brush strokes because they have to keep redipping the chalk into the paint. This gives the feeling of VanGogh's style.
What we have learned is to keep the paint fairly neutral because it will mix with the chalk and make new colors. I have also learned that if you want a lighter painting you need to use white paint with your darker chalk colors and darker paint colors with light chalk if you want a darker painting.
Students are encouraged to have fun with the brush strokes and change their direction or use it in their favor depending on what type of picture they are creating.