Since https://invent.kde.org/plasma/libplasma/-/merge_requests/1214, accent colors follow the color scheme. Using Breeze Dark with dark tasbar, that would be blue... in a blue background (when app is selected). I believe that being green was actually a good idea because it gave more contrast to the fact that there are multiple windows of the same app open. Now that it follows the accent color the contrast is lost. (To clarify, the contrast is lost only when the selected app has multiple windows. But also not too good when not selected.) The screenshot from the MR examplifies this: Before - https://invent.kde.org/-/project/2485/uploads/73bfe7a4c52bbf84ffd1d080baea2967/before-breezedark.png After - https://invent.kde.org/-/project/2485/uploads/31ae7f2c5f1dd144027a0eeec322ca86/after-breezedark.png Notice in those screenshots that being green gave it more contrast, which the author of the MR even said: "[it] is pretty eye-catching" — which I believe was the intention behind it. I think that the change should be reverted and/or we should use some other color or add anything that can give more contrast (maybe an outline to the group indicator icon?). Thanks.
Added Nate to CC list since he was the one to merge the MR.
At this point I think we should consider the current appearance intentional, rather than flipping between the same two states out of indecision, or worse, making it configurable. In principle, this doesn't really need to be attention-getting and have high contrast. The fact that a task is grouped isn't really a high-priority piece of information you need to have shoved in your face. A subtle indicator is enough IMO.