I vaguely remembered having seen a tutorial for a moleskin pencil pocket a while ago. After a bit of googling I managed to find it again here. Thank you Krommama. I made my version with a whole piece of fabric for the pocket, and attached some selvages and a scrap with the text "for colour" on it. That seemed fitting for this book, especially as there turned out to be room for three watercolour pencils in the pocket next to the drawing pen. So there you have it. A few minutes by the sewing machine and a handy accessory was made. Very satisfying.