Hi and welcome to the Forum! Sorry your post didn't get a response sooner.
Honestly, your cabbage looks pretty OK to me. It is natural for the oldest leaves at the bottom to age and wither away.
If it is not quite as green as it could be, that can be an indication of a couple things, over-watering or nutrient deficiency. How (and how often) have you been watering? How fertile is your soil and have you been adding anything to enrich or fertilize it?