Some stamps are more solid. Some stamps are outlines. The stamps with the outlines can be colored in. There are water base markers. There are pencils - regular color pencils and water color pencils. There are blender pens that you can put water or alcohol in to do the blending of the colors. There are Alcohol Markers.
Close to My Heart has ShinHanart Touch Twin Markers. Alcohol markers like water color pencils can be blended. Unil pencils, you can go over the alcohol markers and make them darkers. There is also a marker that will take some of the color off. The Close to My Heart paper is strong enough to go over several times leaving just the desired color that you want. The Touch Win Marker has a small side and then a chisel side which is about 1/4 of an inch of marker to help with the larger parts that you color.
You can use them as background to make the stamp look like there are shadows. Try and see what you can do!