The flight from conversation what is the purpose in using this allusion

The placebo effect.

Explanation:

A placebo effect can be defined as a phenomenon that acts like a treatment and offers benefit in health but in reality is fake. A placebo effect can be defined as anything that looks like to be a medical treatment, but in reality it is not. Though the treatment given is not real, yet it have positive affect on the body. In this treatment, mind is tricked by fake-treatment.

In the given case, Tracy is being given the 'green medicine' that was nothing but water with green food coloring. Though this treatment is not real but gave benefit in Tracy's health.

So, the correct  answer is the placebo effect.