When plugged into a powered USB port, Tiller powers up and performs a self-check. If the computer isn’t running the Tiller software, this results in a flash of red, green, then blue light as the LED is self-checked. If the Tiller software is on and communicating with the Tiller device, the device flashes a white light twice.
If the device detects a fault, it will flash a red light repeatedly until the fault is rectified. On occasion, this may be triggered by a condition with the USB port on your device not providing correct voltage.
Finally, if you're timing, Tiller will show a light corresponding to the current timer colour.