HI I'm trying to define a certain writing technique to aid me in my search of a series of books I read over 10 years ago.

The series in question was written by several authors each author wrote a sub series of 5-10 books each of these sub series took place in the same place but pertained to a different set of characters

So far I have been calling this a "multi-series" but this hasn't been giving me any results close to what I'm looking for

If anyone actually knows the books I'm looking for that would obviously be a better answer

I don't remember much else about them except...
-the unique writing process
-medieval/fantasy genre(young adults)
-each series was color coded using the spine and border of the cover
-each cover looked relatively close to each other---a group of adventurers walking in the distance with a mountain/forest/castle in the background
-I read these books in 2005(I believed them to be fairly new at the time)
-this "multi-series" had more volumes than any of the series posted on lists of "longest fantasy book series" I could find

Again I'm just trying to define the writing process used here but why not kill 2 birds with 1 stone