I'm no expert on The High Chapparel,but I have never heard of it being called The Green Chapparel.
I have seen this episode and the reference to The Green Chapparel passed me by.My guess is that the Diagnosis Murder team wanted to reference The High Chapparel having cast Henry Darrow as an actor in famous TV western series,but couldn't get clearance to use the series name or character name,hence the changes.The High Chapparel was produced by NBC while Diagnosis Murder was produced by Fred Silverman Productions and first aired on CBs.
If you want to research it further there are some good sites for TV westerns and IMDb should have something listed.