Help with broccoli and cabbage plants
Overall the plants look good. The yellow and white color on the old leaves looks like possible cold damage or a type of environmental stress. Remove the old affected foliage.
In general, various larvae can affect these vegetables but you may want to suspect the imported cabbage worm. Look for cabbage butterflies in the area. Monitor and look for signs/symptoms of this pest. You can cover with floating row cover to prevent egg laying but you do not want to trap them underneath the row cover.