I can't do this for you but to describe it more fully for someone wanting to make the skintone, the markings are not "lines of colour... from a line that travels up the spine" but animal markings. The markings on various Specks are described as "mountain cat", "trout" and various other animals - both mammal and piscine. They are not tribal but animal markings.
Thank you for adding to the description, I've only read the first book so far so any other details I have missed out on that appear in later books please add them here!