Mwahahahaha!
On a serious note, I think the color distinction within nagura types relates to color and softness. My Koma is richly stripped with yellow and produces a Splenda colored slurry very very quickly, it is by far the fastest and softest stone I have. The other three have NO yellow, are of a different color grade, and are all harder.
I'm so exciting about honing now that I have these stones. Initially amazing results that will only get better with time. But that's why we do this I suppose