you can pretty much use anything, I have some acrylic paints from local stores. you can also use smooch, ink pads...sometimes you may need to apply more than one coat. you can also cover them in pp or cardstock if you're willing to take the time to cut/sand around the edges. have fun!
I love painting chipboard! I just use cheap acrylic paint from a craft store. I also use glimmer mist, cardstock, and patterend paper on them. I've used ink but I don't like the color of it. Of course, I usually am using the "brown" chipboard so you don't get the true color of the ink. So I stick with paint.