That difference between "glistening" and "dull" is exactly what I hoped you would see. Junichi (and others) have shown you what good skin texture looks like with a vibrant sheen to it. I've shown you the opposite with that little one I pointed out and the dull sanke as well. I've been growing these out specifically to record the way good skin and bad develop over a 1 year time period. Obviously I didn't keep them because they are great fish. Just good to learn about skin and color development.
In those 2 pictures you see 3 different types of skin with different looks to them. I've had these since the day they were born and have grown them out just to watch the different skin qualities either build or deteriorate. The dull kohaku used to have 3 hi plates. One is almost completely gone, one is 90% gone, and the head plate has gone down about 50%. I fully expect it to be shiro muji by summer. The skin has no holding power, and will never get better.
The big Sanke in the second pic shows another result of poor skin. It isn't as lifeless as the little Kohaku. It is more of a "hard" cottony skin type. The beni has no sheen to it and is almost a burnt orange color. The sumi is hard, and may last a year or two, but will never have the sheen to it that the other 2 in that bowl have. Neither of the two to the right are very good fish, but their skin quality has a sheen to it that allows the color to build greater depth and luster. They're just mediocre pond fish, but they'll hold their color much longer.