Hi Rick, thanks for the question.

What you are seeing is most likely varietal differences, problems due to temperature and/or time to harvest. Luna likes cooler temperatures and may do better when planted for fall harvest.  Stressors like extreme weather changes, poor soil, pests, or nutrient deficiencies can cause broccoli to turn yellow prematurely.  Quick to mature cultivars are less likely to have this problem.  If you haven't had a soil test (Home | MSU Soil Test) in a while, it may point out any nutrient issues. Check out the following links:

broccoli2011.pdf (msu.edu)

Growing broccoli in home gardens | UMN Extension

I hope this is helpful.