A great plugin. Thanks Evan.
While using it, I thought about using a picker to help set the range.
In other words you bring up the dialog, then use a picker on the image to get a first cut of the colour range to target. Adjust manually after that. (If the picked colours were overlaid on the colour wheel it would help.)
I'm not familiar with the PDN API and how it can be accessed from plugin code. Has this sort of thing been done before? If it has I'd be prepared to spend a little time trying to integrate it into the code that Evan has generously provided.