The postcards will be printed on a compact 300gsm paper/ card, and uncoated. I've experimented with biro, fountain pen, felt tip, highlighter and marker pens and they all look good without them bleeding.
But what colour should the printed ink be? The image shows the linework in a dark chocolate brown which was chosen to match the parchment background colour, as seen on the sketchbook page image here. But for white colouring cards I'm thinking a black would be best as it is neutral. You can see this in the next set of postcards, next post.