When the headset is charging, it will flash, which means it is not fully charged.
When the light goes to solid red, that means it is still in a charging state (attached with a cord or in it's cradle), but the headset has been fully charged.
Basically, instead of having a green light, when it is fully charged, it shows a solid red light, which means it is charged, but is still attached to a charging device.
The same is true for your controllers. When they are charging, the light flashes. When they are fully charged but still attached to a charging device, they will go to solid red.
Think of the flashing red as charging, and the solid red as a green light, which means it is fully charged.
To make it simple: When the red light is flashing, the headset is charging. When the red light goes solid red, it is done charging.