HOWTO: Factory reset a Hue bulb

Method 1 - Use a Philips Hue Dimmer Switch

1. Turn the bulb on (ensure there are no others in the same lamp, as they could be reset too, using this method). It is important that the bulb has just been turned on. If the bulbs are connected to power for some time already, disconnect them, wait for a second and connect them again.
2. Hold the remote within 4 inches of the bulb.
3. Press and hold the 1st (On / I in some countries) and the and 4th (Off / O in some countries) buttons simultaneously for about 10 seconds, until the bulb starts blinking on and off.
4. If the bulb stopped blinking (it will normally turn fully on again after reset) and the green LED on the remote (top left of the remote) blinked green - your bulb is reset. It's very tempting to release the buttons when the bulb starts blinking. Don't do that! You must keep the buttons pressed until the bulb turns on continuously.

This method can be used for nearly ALL ZigBee light bulbs (confirmed for IKEA Tradfri and OSRAM) and some other ZigBee devices (confirmed for OSRAM ZigBee Plugs).

Method 2 - Use a Lutron LZL-4B Connected Bulb Remote Control

It appears that this device has been discontinued. I managed to find one at Home Depot. It looks like this:

1. Hold the remote within 4 inches of the bulb.
2. Press and hold the 2nd (up arrow) and 4th (light off) buttons simultaneously.
3. The bulb should blink on and off
4. The green LED on the remote should come on and may blink a few times.
5. Continue to hold both buttons on the remote until the green LED goes off completely.

Method 3 - Use the hue-thief

For this method, you will need a zigbee compatible usb key and a linux. Follow the instruction on the README of the project: https://github.com/vanviegen/hue-thief.