For this model I roughly sketched what I wanted the character to look like in pen and ink, including the general idea of the textures. Then I created a grid pattern of black and white textures to cover the various surfaces of the character in blender in the UV editor, and then once I got everything kind of lined up how I wanted it, I baked it onto one image and then touched it up with texture painting and krita+gimp.
A lot of the color initially came from me just using direct color manipulation of the black and white textures, and then painting in more details/gradients to add detail
There is some posterization and dithering I did in gimp as well I believe. But I made everything with the idea that most of the detail is dark and with sharp black edges.