The little tab beside each device is not a battery indicator. It’s the distance to the device from your current location.
Find My iPhone: Locate your device
Green means it’s online, gray means offline. I’ve never seen red before and it’s not mentioned in the support article in the link above.
I’ve never seen the little battery indicator in the upper left be anything but gray. If it’s yellow or red, that is what find my iPhone has for its status. If it’s offline, that status may be out of date?