[cxrlosfx]

>The reason "lila" wasn't used to describe the cabbage is that the German language lacked a word for it.

This is totally it, some people just decided to name it the way they saw it, and depending on the region were they lived.

Rotblaukohl was probably the middle ground, but there's in German a rule to join two colors: das blaurote Kleid (the blue-red dress), in this case it would mean the dress has two colors blue and red.

I might be very far from my area of expertise TBH.