In Germany we don't have kale all year, only in winter. I am substituting it with Savoy cabbage.
I am wondering if I should use the outer leaves which are very green or just the inside. Also are there any drawbacks of using outer leaves?
The outer leaves would be a better substitute for kale. My reasoning is that the kale plant is fully exposed to light. With the cabbage only the outer leaves get to enjoy the sun, thus the greenness. I would say flavour wise the outer leaves come closer to kale too. The cabbage core tends to be sweeter.
You should uses the inner leaves. The outer leaves if not washed correctly can leave insecticides and other fertilizers on the outside. The outer leaves otherwise are okay to use. To properly clean the leaves place the cabbage in a pot for 10 minutes soaking the cabbage. Rinse and repeat. After the second soaking scrub the leaves lightly. There you go all the leaves are now usable.