If you have followed my reports of confusion in Finder Accent and Highlight colours, I have finally (I think) got to the bottom of them.
In some circumstances, such as changing between different Accent colours including grey in the General pane, Finder does become confused and uses a third colour, which is neither of those you have selected.
Although I appreciate the suggestions that these might somehow be generated by colour mixing for creative effect, they’re not. The appearance of purple in various features when my Accent is set to blue and my Highlight to orange is a simple bug. Despite repeatedly setting both the Accent and Highlight colours, I simply couldn’t shift the purple, which appeared as the outline for the edit box within a selection, and as the colour of the progress bar when copying folders in the Finder.
Eventually I did restore order, and can confirm that the Finder doesn’t imagine its own mixed colour different to those set in Accent and Highlights. Following a restart, all those places where purple had shown before now appeared blue, as expected with that as my Accent colour.
You probably don’t have to restart the whole Mac to correct this, just force the Finder to restart instead.
These two enlarged screenshots do show a curious and, I think, distracting effect. Because the edit box outline is semi-transparent, it changes colour according to the background of that part of the window. Where that is the orange highlight, this looks perfectly good. But where that overlies white text in the filename of the item below, it looks quite odd, to me at least. But then I’m only a user, and not a designer.
You’ll also notice that one of the other files listed has its progress circle stuck – another Finder bug which I have detailed here.
Hopefully one of Apple’s engineers understands how a third colour not set by the user gets into situations where the Accent colour should be used, and will fix that. But I’ll not hold my breath.