Colored pencils are about the easiest art material to get hold of, and they don’t have to be expensive. Many brands have excellent student-and kids-quality colored pencils that are sufficient to get started. Wait until you know you’ll use them before investing in an expensive set.
Colored pencil are a fantastic way to add color to your Zentangle art; because they can be sharpened to a fine point, they’re also great for detail. Each of the big brands have their pros and cons: Prismacolor pencils blend superbly, but I find the leads break easily if I apply any pressure. Faber-Castell are beautiful but expensive. Derwent are also very good but can sometimes be a little too waxy. It’s all about personal preferences. Play with each (all can be purchased singularly) and get a feel for what works best for you. You really only need a small set (maybe twenty-four) to get started.