Oh I love the stuff
!!!!!
Hmmm...grey!? Perhaps you have over cooked it?
My asturian cabbage is lovely zingy green on the plant...and doesn't really change that much during cooking, or depending how much you cook it.
To get best out of it, I like growing mine VERY lush...so the leaves are BIIIIG and cook it as soon as it is picked or it soon goes limp and its not so nice anymore.
Young leaves are nice cut thinly or just ripped apart and quick cooked in teeny amount of water..after which I just sprinkle bit of salt, add 'you know what' amount of butter and touch of black & white pepper..and that's it!
Larger older leaves usually end up for chickens...I tend to grow too much of it anyway, so can't keep up with the plants. But I have used them as 'cabbage leaves',,,dipped them into boiling water to soften them up and used them for wrapping other food things which are then cooked in oven like thick cigars...+ sprinkle of cheese of course!