color bars under taskbar icons

Does anyone here know what the little color bars mean that appear under the open program icons in the taskbar? I have noticed that the length of the color bar is longer under the icon of the program I am currently using and short under the icons of the programs that are in the 'background', so I am guessing that is what the different lengths mean? But I am curious to know what the different colors mean. I currently have 3 open programs and they each have different colors - my outlook taskbar icon has a red color bar, my browser icon has a blue one, and my file/folder explorer icon is gray. Also, the outlook color bar always stay red, whether I am currently using it or it is in the 'background', but the colors of the other two (my browser and file explorer icons) change colors depending on which one of them is being used at the time - like right now since I am using my browser, the color bar under the icon is longer and blue, while the color bar under the file explorer is short and gray. But if I switch to the file explorer program, the color bar under that icon changes to long and blue and the color bar under the browser icon (which would now be in the 'background') would change to short and gray. I am concerned that the red color bar could indicate an issue with that program. Any confirmation/clarification on this would be greatly appreciated! Thanks!

So, just a quick follow up to my first post... out of curiosity, I just closed/reopened outlook and now the color of the little color bar is switching between blue and gray depending on if I am currently using that program - blue when I am and gray when it is in the 'background'. Still curious to know why it would have turned to red. ty

The colors I see match the colors I have set for active and inactive caption in the registry. Whichever window is in focus is the active window (darker color longer line). I use a theme I built myself, and I then altered the colors of the different window aspects using Wintools Colorchanger. This changes the registry entries in [HKEY_CURRENT_USER\Control Panel\Colors]. If you are using a pre-built theme, the colors you see are probably built into the theme. You can prove that is the case by changing themes.

I've used that little standalone app for years because I'm always playing with changing the way things look.

All mine are the same color, a light gray.

Thanx for that info glasskuter. I haven't had the time to fully explore the various personalization options for windows 11. I did just now take a look at the various accent color options, which, it turns out, do change the color of the bar under the icons.

I am curious to know why the color under the outlook icon changed to red when I never made a change to the accent color option (until just now), and then, when I closed and reopened outlook, it changed back to the color that was set for it by default when I installed win11, which was blue (I have changed it now to match my background).

I still wonder if it turned red on its own to indicate some sort of issue with outlook (and the red color was present whether or not I was actively using outlook). Anyone have any ideas on that?