Most people either prefer markers or colored pencils to work within their coloring books for adults but both of those groups also like to supplement their work with gel pens, particularly if you want to highlight an area with a glitter or metallic effect. For the lift of markers, you should store them horizontally so the ink stays evenly distributed and has a consistent flow. Gel pens are exceptions and should be stored vertically with tips pointing up. The bronze, browns, and golds in this set are spectacular to use on metal objects in your coloring and blend really well. I find this wet enough that if you are quick and hold both pens in your hand you can blend them as you work being VERY careful not to smear of course They work spectacularly well with the techniques that Cher Kauffman shares in the video at the end of this article for intermediate to advanced gel pen coloring techniques. If you happen to live near a Costco check to see if they are carrying these pens.
Gel Pens - Adult Coloring Book Supplies for Addicted Colorists
Adult coloring enthusiasts tend to look for colored pencils as their coloring tool of choice. And why not? They lay on dry, are relatively cheap, come in a wide variety of colors, etc. One unique attribute of gel pens is the tip remains the same size unlike a crayon or colored pencil that changes shape as you color.