Are you already familiar with rainbow gel pens? They are otherwise normal gel pens, but one pen contains several colors.
When you color with a rainbow pen, you can never be sure when and how the tone changes.
There is no point even trying to design the colors of the image. Wonderfully relaxing, since there is no need to think about the color choices!
Rainbow gel pens are particularly suitable for abstract mandalas and patterns. All the examples here are colored with one or two gel pens only.
Pens: Rainbow gel pens
Book: Mental Images vol 1