Blinking yellow (DFU mode) means the device is waiting for a software update by USB.

You can enter DFU mode:

• By button press (Hold down MODE or SETUP, tap RESET, then continue to hold down MODE until the status LED blinks yellow, then release)
• By USB request (including the CLI particle usb dfu)
• By connecting by USB serial at the magic baud rate of 28800 baud
• By connecting by serial at the magic baud rate 14400 baud
• The firmware on the device has manually entered DFU mode using System.dfu().
• If the device does not have a valid copy of Device OS, or does not have user firmware flashed to it

Once in DFU mode, you can flash binaries using the Particle CLI or you can use Device Restore to recover a device that is stuck in DFU mode.