Diagnosis
The bulb has likely crashed or lost its Zigbee connection to the bridge. A physical power cycle is required to reset it.
How to Fix It
- Turn off the light fixture at the wall switch for a full 10 seconds.
- Turn it back on. The bulb should flash once, then return to a default warm white.
- Open the Hue app, go to Settings > Lights, and select the problematic bulb. Choose "Delete."
- Then, go to Settings > Add light. The app will search; turn the wall switch off and on once to make the bulb flash. It should be found.
If That Didn't Work
The bulb might be in a faulty socket or on a dimmer switch (incompatible). Try it in a different, standard lamp socket to test.
Heads Up
Never use smart bulbs on circuits with physical dimmer switches unless the dimmer is a compatible smart dimmer (like Hue's own). It will fry the bulb's electronics.