All objects have a shape, correct? Cubes, spheres, cylinders, or strange organic shapes that are hard to define.
And all three dimensional shapes have edges, right? The sides, tops, and bottoms.
As those edges get further away from us, the color desaturates- it gets grayer and less brilliant, less pure.
So all of those Copic formulas telling you to color the edges of an object one or two steps darker? They're telling you to do the wrong thing!
Copic says to ADD more color when you should be using LESS color.
And that's why your coloring isn't getting more realistic.
You can practice and practice and practice... but if you're always "shading" using Copic math, you are never going to add any realism.
Artists do not "shade". We use our eyes and our powers of observation to determine what the color is doing as it moves away from us. Then we duplicate what we see on our projects. That's Color Sculpting.