Using e.g. the local Midnight style if there is only on Groupy tab in Explorer, it is colored blue like the first tab of Explorer itself. As soon as there is a second tab, the active Groupy tab is black and the inactive Groupy tab is darkgrey. Then, if Explorer is not focused, the active Groupy tab goes darkgrey with a blue line on top, the inactive darkgrey with no line. There is no logic is the way, the colors change. I suppose, the way those Stardock apps play together is not quite finished yet...

In this case, I'd expect that - focused window or not, single tab or not - the active tab always would have a blue line and both tabs the same base color.

In general I would expect Groupy tabs to always follow the chosen Curtains style, since their styling is accessible from within another Stardock app.