A black ink pad is all you need to get started, and one is included in the 1st edition box set by default.
Ink pads are also available in a variety of colors. Here are some examples, using symbols from the 1st edition of the Trailblazer set.
Two types of ink pads are available in the shop:
The rectangular ink pads contain archival dye ink, which is fast-drying and permanent. These should be used with stamps to create symbols of terrain.
The smaller square ink pads contain water-based ink. These should be used with blending tools to create colored backgrounds, or with stencils to create homemade hex paper.