Active Apps widget - Sometimes green/sometimes red

[KJIsaacson]

I've placed the Active Apps widget on my home screen, and I see that it usually shows a green bar at the bottom, but sometimes shows a red bar. What's the significance? (FWIW, there's usually only 1 or 2 apps shown to be running, so the color changes doesn't seem to be related to the number of apps.)

Thanks.
Ken