Primary school coloring is all about the ability to fill in shapes with color. A 2D shape on paper remains 2D when it is filled evenly with color. More advanced coloring is about making 2D shapes appear to be 3D.
Colored pencil is an ideal way to do this. Remember to slow down and use no pressure as you manipulate color into 3D shapes. The sample shown here contains simple abstract shapes, and can be found here.
Start by filling a shape very lightly with the desired color with soft strokes. The lightest part of the shape is closest to the viewer, and the darkest are farther away. Gradually darken the receding edges of the shape using the same or related colors.
Starting dark and working light just doesn’t seem to work as well.