Sorry for the delay, lately I am really busy with my master at university... Read read read read... Anyways.
To answer your question, I thought many times of adding some colours, may it be for some shading or texture or for borders. The problem is this is a style I made for a civilization in a novel project of mine. The main goal is to be accurate, not artistic though looking good is also a plus. Also, if I add colours, I will need to find the correct pens. All of it is hand made, the only exceptions is the writting you will find early in the posts of this thread. Besides that, hand made. Many times I tried to get my hand on some precise pigment but to no avail.