Ensure you have downloaded the latest version of the Boost Juice App on your mobile device.
Open the Boost Juice App and click on the Vibe tab. There is a $2 off challenge where you have to pick the correct answer out of 4 possible choices.
Answer: blue
Posting this to help others who might already be buying a Boost to save a couple of bucks, not for my own benefit.
Usually someone will jump in pretty quickly and post the answer in the comments.