Gel pens are popular for colouring with both children and adults alike. The small nib tips and gel ink of gel pens smooths over the paper, giving you better and thicker-looking lines.
Although they are similar to a rollerball pen, gel pens differ in one way. As the name suggests, gel pens use gel ink, which is solid until pressure is applied and it becomes liquid. This makes the pen less likely to dry out and consistently draws thicker lines. But with only light pressure, the finish can be patchy and pressure is needed for the ink to flow.