Hey Weltenlaeufer, sorry for the late reply. I had to scroll way back to find what you were talking about when you mentioned the "flow"of the capes.
Indeed, I think they are difficult. They have a double light gradient: One goes from inside the fold (dark) to outside the fold (light). The other one goes from top and outstanding parts (light) to bottom or inward bent parts (dark).
That's why I put black primer, then white primer from the top. Here you have already already parts standing out that should stand out, however most of the folds are equally bright on the outside and inside.
Next I apply the color of the cape, thinly so that the two-color-base still shines through, then I use a wash or I make a wash with medium and make the inside of the folds darker, the outside brighter.
Then to challenge myself, I sometimes paint a picture on the cloak. Back in the day I started with a line going round the base of the cloak, always trying to follow that awesome smooth gradient that the airbrush made, later I made more complicated motives. All of them work great
Today I made an Ork. I tried True Metal the first time on the sword. I will try to upload it later the day.